Toward High-Performance p-Type Two-Dimensional Field Effect Transistors: Contact Engineering, Scaling, and Doping

Abstract

n-type field effect transistors (FETs) based on two-dimensional (2D) transition-metal dichalcogenides (TMDs) such as MoS₂ and WS₂ have come close to meeting the requirements set forth in the International Roadmap for Devices and Systems (IRDS). However, p-type 2D FETs are dramatically lagging behind in meeting performance standards. Here, we adopt a three-pronged approach that includes contact engineering, channel length (<i>L</i>_ch) scaling, and monolayer doping to achieve high performance p-type FETs based on synthetic WSe₂. Using electrical measurements backed by atomistic imaging and rigorous analysis, Pd was identified as the favorable contact metal for WSe₂ owing to better epitaxy, larger grain size, and higher compressive strain, leading to a lower Schottky barrier height. While the ON-state performance of Pd-contacted WSe₂ FETs was improved by ∼10× by aggressively scaling <i>L</i>_ch from 1 μm down to ∼20 nm, ultrascaled FETs were found to be contact limited. To reduce the contact resistance, monolayer tungsten oxyselenide (WO_<i>x</i>Se_<i>y</i>) obtained using self-limiting oxidation of bilayer WSe₂ was used as a p-type dopant. This led to ∼5× improvement in the ON-state performance and ∼9× reduction in the contact resistance. We were able to achieve a median ON-state current as high as ∼10 μA/μm for ultrascaled and doped p-type WSe₂ FETs with Pd contacts. We also show the applicability of our monolayer doping strategy to other 2D materials such as MoS₂, MoTe₂, and MoSe₂.