Density functional analysis of oxide dipole layer voltage shifts in high κ/metal gate stacks.

The mechanism of gate threshold voltage (V_T) shifts observed in high κ/metal gate stacks is investigated by a density functional theory. This finds that V_T depends on the band alignments and the chemical trends between the component oxide layers, such as HfO₂, SrO, La₂O₃, Al₂O₃, and SiO₂. Based on the electron counting rule, we have built three insulating SiO₂/SrO, SiO₂/La₂O₃, and SiO₂/Al₂O₃ interfaces, all of which feature a clean bandgap. Two methods have been adopted to derive the band alignments between these four oxides, which are consistent with each other. The results show staggered, "staircase" band alignments and enable La₂O₃ and Al₂O₃ layers to shift the metal electrode Fermi level in opposite directions and to approach the Si conduction band and valence band edge positions, respectively. This analysis updates previous empirical models of this effect based on metal oxide ion densities or electronegativity scales and confirms that the oxide layer scheme is suitable for controlling the effective metal work functions in metal–oxide–semiconductor field-effect transistors. [ABSTRACT FROM AUTHOR]