Effect of oxygen partial pressure on the in situ growth of Y-Ba-Cu-O thin films on SrTiO₃

The evolution of YBCO film growth with thickness at various oxygen pressures was observed by in situ synchrotron x-ray diffraction in real time. When the films were deposited at 2 Å/s and 730°C under higher oxygen partial pressures (in an Ar/O₂ mixture of 90 mTorr), the nucleation was observed to have c-axis orientation. After the films reached a critical thickness, the growth of the YBCO film changed from c axis to a axis and then propagated epitaxially. This provides evidence that a-axis epitaxial growth nucleates on a c-axis base. The critical thickness reflects the competition between the growth of the c and a axes, which is determined by the oxygen partial pressure in the process of thin-film formation. The a-axis oriented films showed a very sharp rocking curve (less than 0.1°) which indicates a very high structural quality. For very low oxygen partial pressures, the in situ growth process was very similar, but the initial nuclei involve a second phase mixed with a small amount of c-axis "123" phase. The nucleation and growth mechanisms of in situ YBCO films are discussed.