We correct the background of the XPS data with the Shirley method and calibrate the XPS peaks by shifting the C 1s peak to 284.8 eV. We characterize the elemental depth profiles with the dynamic Secondary Ion Mass Spectrometry (SIMS) equipment (Hiden Analytical; Warrington, UK) employing a continuous Ar + beam with 4 keV energy under ultra-high vacuum conditions of below 10 -9 Torr. The square-shaped spot size for ion beam etching on samples is around 0.5-1.0 mm.