Atomic mechanism of flat-band voltage shifts by La2O3 and Al2O3 in gate stacks

The origin of the flat-band voltage shifts for La- and Al-based oxide capping layers in high k metal gate stacks is studied by ab initio calculations on atomic models. Substitutional La, Al, Sr, and Nb at the HfO2–SiO2 interface create dipoles, which shift the flat band voltage in the experimentally observed direction, negative for La and Sr and positive for Al and Nb. The shift does not correlate with the metal’s valence, being opposite for La and Al, which rules out a vacancy model. The shift does correlate with the metal electronegativity/work function. It does not correlate with oxygen ion density, as this is not varied.