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This paper discusses a role of the oxygen vacancy in HfO2/ultra-thin (UT) interfacial layer (IL) SiO2 gate stacks, focusing on the VFB roll-off. The metal/top-SiO2/HfO2/UT IL-SiO2/Si gate stacks have been studied. It is found for the first time that the VFB roll-off is eliminated by inserting 1~2 nm top-SiO2 between metal gate and HfO2. This elimination of the VFB roll-off is explained by compensating the bottom dipoles at HfO2/IL-SiO2 interface with the counter dipoles at top-SiO2/HfO2 interface. Therefore it is concluded that the bottom dipole is assigned to the dominant origin of the VFB roll-off.