Chemical doping for threshold control and contact resistance reduction in graphene and MoS2 field effect transistors

Two-dimensional materials such as graphene and MoS₂ are promising materials for a wide range of electronic and photonic applications. Graphene has extremely high mobility, tunable optical absorption and strong quantum capacitance making it interesting for high-speed field-effect devices [1], optical modulators [2], and wireless sensors [3]. MoS₂ has a 1.6-1.8 eV band gap and reasonable mobility, making it interesting for scaled logic devices [4]. In all transistors, doping is an essential element necessary for controlling the threshold voltage and minimizing extrinsic resistances. However, both graphene and MoS₂ suffer from the difficulty of achieving chemical doping. In situ doping of graphene during growth [5] has been demonstrated, but limits the ability to spatially control doping, while electrostatic doping with gate electrodes [6] is unrealistic for practical circuit applications. Therefore, spin-on chemical doping emerges as an attractive method to control doping in 2D materials since it can be controlled spatially and candidate dopants for achieving both n-type and p-type doping [7,8] have been identified. Prior work on spin-on doping for graphene has involved either substrate-gated devices or the extension regions of FETs, but not involved threshold voltage control in practical device geometries [9,10]. To our knowledge, no reports of chemical doping in MoS₂ have been reported. In this abstract, we report on two key aspects of chemical doping in 2D transistors. First, we demonstrate spin-on chemical doping using PEI of graphene FETs (gFETs) with local metal back gates and HfO₂ gate dielectrics and show that the “natural” p-type doping can be overcome to reproducibly create n-type gFETs that operate in air. We further demonstrate n-type chemical doping of bi-layer MoS₂ for the first time and show evidence of reduced contact resistance to buried metal electrodes.