From the inset of Fig. 1, we can see that the position of peak (220) in curve (c) is shifted to a lower angle relative to that in curve (b), which indicates that the lattice constant is increased. The variation of lattice parameters may be caused by the strong surface strain or some impurity defects. Therefore, the XRD results show that oxygen pressure in the process of deposition actually has a great influence on the structure of the samples.