title: Dopant morphology as the factor limiting graphene conductivity.
abstract: Graphene's low intrinsic carrier concentration necessitates extrinsic doping to enhance its conductivity and improve its performance for application as electrodes or transparent conductors. Despite this importance limited knowledge of the doping process at application-relevant conditions exists. Employing in-situ carrier transport and Raman characterization of different dopants, we here explore the fundamental mechanisms limiting the effectiveness of doping at different doping levels. Three distinct transport regimes for increasing dopant concentration could be identified. First the agglomeration of dopants into clusters provides a route to increase the graphene conductivity through formation of ordered scatterers. As the cluster grows, the charge transfer efficiency between graphene and additional dopants decreases due to emerging polarization effects. Finally, large dopant clusters hinder the carrier motion and cause percolative transport that leads to an unexpected change of the Hall effect. The presented results help identifying the range of beneficial doping density and guide the choice of suitable dopants for graphene's future applications.
authors.affiliation: Department of Material Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan.
authors.forename: Mario
authors.initials: M
authors.lastname: Hofmann
authors.affiliation: Graduate Institute of Opto-Mechatronics, National Chung Cheng University, 168 University Road, Min-Hsiung Township, Chiayi County 62102, Taiwan.
authors.forename: Ya-Ping
authors.initials: YP
authors.lastname: Hsieh
authors.affiliation: Department of Material Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan.
authors.forename: Kai-Wen
authors.initials: KW
authors.lastname: Chang
authors.affiliation: Graduate Institute of Opto-Mechatronics, National Chung Cheng University, 168 University Road, Min-Hsiung Township, Chiayi County 62102, Taiwan.
authors.forename: He-Guang
authors.initials: HG
authors.lastname: Tsai
authors.affiliation: Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan, ROC.
authors.forename: Tzung-Te
authors.initials: TT
authors.lastname: Chen