In situ XPS was performed with an Omicron XM-1000 monochromated Al-Kα source, with the pass energy to the detector set to 20 eV. The angle of the surface normal with respect to the detector was 1°. The method to determine silicon oxide thickness was similar to the 5P* method described by Seah and Spencer39. An R0 value of 0.80 was determined by measuring silicon substrates with different thicknesses of thermally grown oxides. An attenuation length of photoelectrons in silicon dioxide of 3.448 nm was used39.