Investigation of the mitigation of charged impurity and defect scattering effects by polar molecules on graphene
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Graphene is a promising material for use in microelectronics, due to its high mobility, operating frequency, and good stability. These characteristics are governed by charge carrier transport and charged impurity scattering effects, the latter of which are caused by adventitious doping from both the ambient and semiconductor fabrication processes. A variety of chemical interactions between graphene and both its defects and charged impurities influence the electrical properties. This dissertation focuses on the chemical interactions between polar molecules and the impurities and defects on graphene. The monatomic thickness of the graphene monolayer renders interfacial charged impurities located at the graphene/substrate interface susceptible to the dielectric environment surrounding graphene. Our group has shown that the electrical properties of graphene devices are improved upon coating with fluoropolymers or exposure to gas-phase polar organic vapors. These improvements include reduction of the Dirac voltage, increased mobility, and decreased residual carrier density. We attribute these changes to screening by polar molecules of fields induced by charged impurities/defects in or near the active layer. The magnitude of the changes produced in the graphene device parameters scales well with the dipole moment of the delivered vapor molecules. These effects are reversible, a unique advantage of working in the vapor phase. The changes observed upon polar molecule delivery are analogous to those produced by depositing and annealing fluoropolymer coatings on graphene. We attribute these changes to similar charge screening phenomena. Quantum mechanical modeling showed that polar molecules interacted more strongly with impure or defective graphene than with pristine graphene. Molecular mechanics simulations revealed more insight into how polar molecules interact with two types of charged impurities atop graphene. Polar molecules both displace and electrostatically screen charged impurities to reduce the electrostatic potential profile in the plane of graphene. These computational observations correlate well with our experimental results to support our hypothesis that polar molecules can act to screen charged impurities on or near monolayer graphene. Such screening favorably mitigates charge scattering, improving graphene transistor performance. Our understanding of charged impurity screening methods can further be applied to graphene nanoribbons and other 2D materials.