Fermi Pinning in Metal-Oxide-Semiconductor Structures Results from Low Oxygen Content at the Metal-Oxide Interface

The selection of a proper metal for replacement of polycrystalline silicon as the metal gate in future generation transistors has been hampered by pinning of the metal Fermi level at the metal/dielectric interface. Using monoclinic hafnia and zirconia as the gate dielectric we compare two different metal gate/gate dielectric interface structures where the oxygen affinity of the metal gate is either low or high under normal processing conditions. We show that for the metal gate with high oxygen affinity (molybdenum) the metal Fermi pinning is weaker than for the metal gate with low oxygen affinity (platinum). Through first principles calculations this behavior is explained as the result of an oxygen-rich interface for the former and a reduced interface for the latter. Based on these observations we propose a general model for Fermi pinning at the metal/metal oxide interface as mainly caused by low levels of interfacial oxygen. Such red uced interfaces may result from processing conditions as well as by the metal gate low affinity for oxygen.