High-speed growth of YBCO films by post-annealing of precursor films

Precursor films of 200 run thickness are deposited on SrTiO3 single crystals by the coevaporation technique using Y, BaF2 and Cu. Precursor films including BaF2 are generally annealed at a low oxygen partial pressure with water vapor. On the contrary, the precursor films in the present process are annealed at a background pressure of 5x10(-4) Torr. Only pure oxygen gas flows near the precursor films, and no water vapor is introduced to the reaction chamber. The maximum specimen temperature is 700 degreesC, which is reached in about 30 minutes. In order to investigate the YBCO film growth, the reflection high-energy electron diffraction (RHEED) patterns are observed during annealing of the precursor films. RHEED images suggest that two polycrystalline phases appear at low temperatures. Then the RHEED image corresponding to the YBCO begins appearing just before the maximum temperature is reached, indicating that the YBCO film grows rapidly. We prepared several kinds of YBCO films keeping them at the maximum temperature for 1, 5 and 30 minutes. As a result, it was determined that the YBCO film growth is almost finished after 5 minutes of annealing, and a longer annealing time than 5 minutes appears to be useful for slightly stronger grain connectivity.