Ab initio study of Al-Ni bilayers on SiO2 : Implications to effective work function modulation in gate stacks

The valence band offsets and the effective work function of metals and metal bilayers on SiO2 were investigated using density functional theory within the local density approximation. Metal-oxide interface effects and the underlying metal thickness in contact with oxide were examined. As model systems both low and high work function metals had been chosen, i.e., aluminum and nickel, and their bilayers on β-quartz. It is found that the effective work function of a metal on oxide is strongly controlled by the interface atomic arrangement, the calculated valence band offset differences leading up to 0.5 eV for interfaces containing excess oxygen. It is also shown that one or two atomic layers of the underlying metal are sufficient to shift the effective work function of bilayers to that of underlying metal.

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