Extension Doping with Low-Resistance Contacts for P-Type Monolayer WSe₂ Field-Effect Transistors

Source/Drain extension doping is crucial for minimizing the series resistance of the ungated channel and reducing the contact resistance of field-effect transistors (FETs) in complementary metal-oxide-semiconductor (CMOS) technology. 2D semiconductors, such as MoS₂ and WSe₂, are promising channel materials for beyond-silicon CMOS. A key challenge is to achieve extension doping for 2D monolayer FETs without damaging the atomically thin material. This work demonstrates extension doping with low-resistance contacts for monolayer WSe₂ p-FETs. Self-limiting oxidation transforms a bilayer WSe₂ into a hetero-bilayer of a high-work-function WO_xSe_y on a monolayer WSe₂. Then, damage-free nanolithography defines an undoped nano-channel, preserving the high on-current of WO_xSe_y-doped FETs while significantly improving their on/off ratio. The insertion of an amorphous WO_xSe_y interlayer under the contacts achieves record-low contact resistances for monolayer WSe₂ over a hole density range of 10¹² to 10¹³ cm^-2 (1.2±0.3 kΩ μm at 10¹³ cm^-2). The WO_xSe_y-doped extension exhibits a sheet resistance as low as 10±1 kΩ □^-1. Monolayer WSe₂ p-FETs with sub-50 nm channel lengths reach a maximum drain current of 154 μA μm^-1 with an on/off ratio of 10⁷-10⁸. These results define strategies for nanometer-scale selective-area doping in 2D FETs and other 2D architectures.