Low Resistance and Stable P-Type Contacts to Monolayer WSe₂ Through Chlorinated Solvent Doping

Forming good quality p-type contacts to two-dimensional (2D) semiconductors such as tungsten diselenide (WSe₂) remains a significant challenge [1]. Although semimetal [2] and transferred metal contacts [3] have been demonstrated to reduce the contact resistance (R_C) in p-type WSe₂ transistors, contact metal engineering using stable and industry-compatible methods often results in a high Schottky barrier at the metal-2D semiconductor interface, preventing low R_C values.

Alternatively, stable p-type substitutional and surface charge transfer doping near the contact region can be used to lower R_C and improve the hole current. Although substitutional doping with electron acceptors (e.g. vanadium [4]) is stable due to the formation of chemical bonds, this method requires multiple material growth steps, making selective-area doping difficult. In comparison, p-type surface charge transfer doping withdraws electrons from the 2D channel using capping layers (e.g. MoO_x [5]) with work function below the Fermi level of the WSe₂ (i.e., high electronegativity). This doping technique does not break the host lattice and introduces few scattering centers. However, the long-term time and thermal stability of these layers remains unclear [6], and this method often results in large off-state currents [5,7].

In this work, we show that doping monolayer WSe₂ transistors with chloroform leads to a significant and stable improvement in their p-type performance that persists over several months. We compare the same set of devices before and after our doping procedure and find that our technique leads to 100× improvement in drain current (I_D) after doping, with hole current reaching up to 203 μA/μm at V_DS = -1 V, large on/off ratios (>10¹⁰) and low contact resistance of 2.5 kΩ-μm, at room temperature. The maximum current density is among the best reported for p-type monolayer WSe₂ and, unlike many charge-transfer doping methods, does not increase the off-state current. Additionally, bilayer WSe₂ devices fabricated and doped using this method reach up to 267 μA/μm at V_DS = -1 V. Remarkably, the chloroform doping is stable over time, retaining a I_D,max value > 96% (median) of the post-treatment measurement after 6 days (and > 72 % after 134 days) at room temperature and after annealing at 150°C in vacuum.

We further investigate the mechanisms of chloroform doping using photoluminescence spectroscopy at 6.7 K, which reveals that the chloroform reduces the intensity of the neutral A exciton emission and increases the prominence of the trion peaks, consistent with the expected p-type doping. Atomic force microscopy confirms that chloroform leaves minimal residue on the WSe₂ and does not disrupt the crystal structure. The potential stability of chloroform doping under cryogenic conditions suggests this approach holds future promise for low R_C in quantum transport devices.

[1] Y. Xiong et al., Adv. Mat. 2206939 (2023)
[2] Y.T Lin et al., Nano Lett. 24, 8880 (2024).
[3] Y. Liu et al., Nat. Electron. 5, 579–585 (2022).
[4] A. Kozhakhmetov et al., Adv. Fun. Mat. 31, 2105252 (2021).
[5] L. Cai et al., Nano Lett. 17, 3854 (2017).
[6] Y. Xiong et al., Adv. Mater. 35, 2206939 (2023).
[7] P.H. Ho et al., Nano Lett. 23, 10236 (2023).