During thin film growth, oxygen content in oxide thin films varies depending on the growth conditions or post-treatments. Three main approaches have been commonly used to accommodate oxygen vacancies: (I) by generating corresponding vacancies in cation sites; (II) by altering the valence state of cations without cation non-stoichiometry; and (III) by incorporating both cation vacancies and change of valence state. Generation of vacancies in cation sites or forming cation-anion vacancy pairs usually occurs in metal oxides without multiple valence states such as ZnO, SrTiO3, BaTiO3, etc19. For example, electronic conduction and superconductivity in SrTiO3 as well as ferroelectricity in BaTiO3 are influenced by oxygen vacancies induced cation stoichiometry202122. It was reported that the cation stoichiometry is strongly affected by the oxygen pressure during synthesis. In both SrTiO3 and BaTiO3, lower oxygen pressure during film deposition results in larger out-of-plane lattice parameter23.