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We have investigated the role of oxygen in Hf-based high-k gate stacks on Vfb shift. It is clearly shown that the Vfb of the HfSiOx-based high-k materials of the weak ionic oxide was almost constant irrespective of the oxidation annealing temperature. On the other hand, the HfO2-based high-k materials of the strong ionic oxide caused the positive Vfb shifts by introducing additional oxygen into high-k films. These suggest that the control of strength of ionic bond and oxygen content due to the oxygen transfer at hetero interface of ionic high-k/covalent SiO2 is a key if it assumes that the Vfb shift occurs dominantly at high-k/SiO2 interface.