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

Abstract

The mechanism of gate threshold voltage (VT) shifts observed in high κ/metal gate stacks is investigated by a density functional theory. This finds that VT 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.