Thickness dependence of carrier mobility in mono- and bi-layer graphene with HfO2 gate dielectric

Graphene, a two-dimensional layer of carbon atoms in a honeycomb lattice, can potentially serve as an alternative channel material for future electronics technology owing to its high (> 10,000 cm²/Vs) intrinsic mobility. Understanding the carrier scattering mechanism in graphene devices with high-λ dielectrics is key to enabling top dielectric-metal stacks that combine a high capacitance and high electron mobility. Here we provide a systematic study of carrier mobility as a function of HfO₂ dielectric thickness, and as a function of temperature. Our results show that the carrier mobility decreases during the deposition of the first 2-4 nm of top dielectric, and remains constant for thicker layers. The carrier mobility dependence on temperature is relatively weak, indicating that phonon scattering does not play a dominant role in degrading the carrier mobility. The results strongly suggest that fixed charged impurities located in close proximity to the graphene are responsible for the mobility degradation. We speculate that positively charged oxygen vacancies, ubiquitous in high-λ dielectrics, are the mobility limiting factor.