Thickness Dependence of Oxygen Vacancy Ordering in Strained LaCoO(3-x )Thin Films

The physical properties of perovskite oxides are very sensitive to oxygen vacancy structures, and an unambiguous acquaintance of vacancy structures of thin films is the precondition for applications of many functional devices. Here, we report critical thickness control over oxygen vacancy ordering structure in epitaxial LaCoO3-x (LCO) thin films characterized by atomic resolution scanning transmission electron microscopy (STEM). Lattice mappings extracted from Cs-corrected STEM images demonstrate that vacancy ordering planes are vertically distributed in LCO films grown on SrTiO3 substrates with the thickness above 5 nm, transversally distributed when the thickness is below 5 nm and distributed in a mixed manner when the thickness is 5 nm. Similar phenomenon also happens in LCO/NdGaO3 film systems with the critical thickness of around 13 nm due to the reduced tensile strain imposed by the substrate. The thickness effect phenomenon is collaborated by first-principles calculations and the formation mechanism is discussed from the point of view of strain relaxation. These findings demonstrate that the thickness can exquisitely control the periodic modulations of vacancy ordering structure in LCO thin films, suggesting an effective pathway of thickness controlling and enhancing physical properties of functional oxide films.