Atomic mechanism of electric dipole formed at high-K: SiO2 interface

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The mechanism of flat-band voltage shifts in La- and Al-based, etc., oxide capping layers in high-K (dielectric constant) metal gate stacks is investigated by ab initio calculations on atomic models. The capping layer dopants are calculated to segregate to the high-K:SiO2 interface in most cases. An interfacial dipole is observed at both the pure HfO2:SiO2 interface and at oxide doped HfO2:SiO2 interfaces by plotting electrostatic potentials perpendicular to the interfaces. Substitutional La, Sr, Al, Nb, and Ti atoms are calculated to induce potential shifts at the HfO2: SiO2 interface which shift the valence band offset in the experimentally observed directions. The shift does not correlate with the metal’s valence, being the opposite for La and Al, which rules out the oxygen vacancy model. The shift does correlate with the metal’s group-electronegativity or metal work function. The potential shift due to A-O and O-A bond dipoles cancels out, on average, in the ‘bulk’ parts of the gate oxide film, and it is only finite where there is a change in the dielectric constant and screening across this buried interface. The net dipole potential shift only comes from those dopant atoms located at the interface itself, not those that diffused away from this interface.