Chemically induced Fermi level pinning effects of high-k dielectrics on graphene

High-k materials such as Al₂O₃ and HfO₂ are widely used as gate dielectrics in graphene devices. However, the effective work function values of metal gate in graphene FET are significantly deviated from their vacuum work function, which is similar to the Fermi level pinning effect observed in silicon MOSFETs with high-k dielectric. The degree of deviation represented by a pinning factor was much worse with HfO₂ (pinning factor (S) = 0.19) than with Al₂O₃ (S = 0.69). We propose that the significant pinning-like behaviors induced by HfO₂ are correlated with the oxygen exchange reactions occurred at the interface of graphene and HfO₂.