Analysis of decomposed layer appearing on the surface of barium strontium titanate

Chemical states and structures of thin degenerated layers grown on the surface of (Ba, Sr)TiO3 (BST) films are investigated by X-ray photoelectron spectroscopy (XPS). It was found that decomposition occurred as a result of the air exposure and also as a result of annealing. A careful study of a Ba 3d peak in the XPS spectrum reveals that Ba in BST crystal easily decomposes out of the perovskite phase and makes BaCO3 or barium oxides, whereas no significant change occurs in Sr, or Ti oxides. This degenerated layer can be removed by nitric acid treatment. It was also found that the surface BST becomes rather stable against exposure to air compared with the original one after the nitric acid treatment.