Origin of the flatband-voltage roll-off phenomenon in metal/high-k gate stacks

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The effect of flatband-voltage reduction [roll-off (R-O)], which limits fabrication options for obtaining the needed band-edge threshold voltage values in transistors with highly scaled metal/high- k dielectric gate stacks, is discussed. The proposed mechanism causing this R-O phenomenon is suggested to be associated with the generation of positively charged oxygen vacancies in the interfacial SiO2 layer next to the Si substrate. The vacancies in the interfacial layer are induced by oxygen outdiffusing into the overlying high- k dielectric. The model is consistent with the variety of observations of R-O dependence on the electrode and substrate type, high- k dielectric composition and thickness, temperature, etc. The model's predictions were experimentally verified. © 2006 IEEE.