Chemical Doping and Interface Engineering of Monolayer TMDs

S. Zhang, C.A. Hacker
National Institute of Standards and Technology,
United States

Keywords: TMDs, doping, FETs, PL, XPS

Summary:

Developing processes to controllably dope transition-metal dichalcogenides (TMDs) and their electrical contact interfaces are critical to achieving commercial viability for optical and electrical applications. For low-power, high-performance complementary logic applications, both n- and p-type materials must be developed. Using different TMDs withing field effect transistors (FETs) for n-FETs and p-FETs is possible but increases the complexity of the device fabrication and scaling. In this work, we successfully expanded the application of electron-transfer doping using molecular reductants and oxidants to various TMDs, including monolayer MoS2, MoSe2, WS2, and WSe2. Notably, both n- and p-type transport properties were achieved in all four monolayer-TMDs and both non-degenerate and degenerate regimes are accessible.[1] The degree of doping can be finely controlled by the choice of dopants, treatment time, and the concentration of doping solution. No previous report demonstrates such strong doping of both n- and p-type and can be used on a wide range of TMDs. Detailed physical characterizations were conducted to study the doping effect and to understand the underlying mechanisms. Photoluminescence (PL) properties of the doped monolayer TMDs were measured; for all four materials, the PL intensity is enhanced with p-doping but reduced with n-doping. Raman spectroscopy was conducted to explore the doping effects on the phonons of the host TMDs. X-ray photoelectron spectroscopies (XPS) were performed to investigate the impact of doping on the TMDs chemical structure and energy levels. The carrier density introduced through doping was calculated and compared from different measurement techniques. Our study also showed that p-type MoS2 can only be achieved by combining molecular doping with high-work function Pd contacts.[2] With low work function metal Ti, only ambipolar behavior can be achieved. It was shown that the contact resistance and Schottky barrier heights can be effectively modulated through doping of the channel materials and the contact engineering by changing the contact metal. A relatively low hole injection barrier of ≈156 meV was obtained for p-doped MoS2 with Pd contacts. A MoS2 inverter based on pristine (n-type) and p-doped monolayer MoS2 was fabricated, demonstrating the potential application of 2D-based complementary electronic devices. In summary, we have demonstrated a solution-based charge transfer doping on several 2D semiconductors. This technique provides a simple yet effective route to tailor the band structure of the 2D materials and control the resulting electrical and optical properties.