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

摘要

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.