A design strategy for high-performance p-type two-dimensional field effect transistors

Abstract Doping plays a critical role in tailoring the characteristics of semiconducting materials and electronic devices. Specifically, in the context of field-effect transistors (FETs), degenerate doping in the silicon channel beneath the source and drain regions has become essential for achieving high-performance n- and p-type devices, as well as significantly reducing contact resistance (RC). In contrast, two-dimensional (2D) semiconductors have mainly relied on metal work-function engineering to lower RC. While this approach has proven successful for n-type 2D FETs due to the natural tendency of the metal Fermi level to align near the conduction band edge, it has been challenging to achieve the same for p-type 2D FETs. To address this, first, we demonstrate that degenerate p-type doping can be accomplished in thick (>3 monolayers) MoSe2 and WSe2 FETs through substitutional doping of transition metal with V, Nb, and Ta. However, the degenerate doping leads to weakened electrostatic gate control, resulting in a poor on/off current ratio. Interestingly, the doping effectiveness is significantly reduced in thinner flakes (< 3 monolayers) due to strong quantum confinement effects, thereby restoring the electrostatic gate control. Based on this observation, we designed a FET structure where the channel is constructed using thinner 2D material, while the contact regions consist of degenerately doped thicker layers, allowing us to achieve both low RC and high on/off current ratio. The doping and device design approach we propose should be just as relevant for synthetic 2D materials and n-type 2D FETs. Our insights may steer the direction of large-scale synthesis methods, prioritizing the creation of doped multilayers over monolayers to further progress in 2D FET technology.