The growth process of GeOx films formed by plasma post-oxidation (PPO) at room temperature (RT) is investigated using angle-resolved x-ray photoelectron spectroscopy (AR-XPS).The experimental results show that the distributions of the Ge4+ states,a mixture of the Ge2+ and Ge3+ states,and the Ge1+ states are localized from the GeOx surface to the GeOx/Ge interface.Moreover,the Ge1+ states are predominant when the two outermost layers of Ge atoms are oxidized.These findings are helpful for establishing in-depth knowledge of the growth mechanism of the GeOx layer and valuable for the optimization of Ge-based gate stacks for future complementary metal-oxide-semiconductor (MOS) field-effect transistor (CMOSFET) devices.