Air Stable Doping and Intrinsic Mobility Enhancement in Monolayer $MoS_{2}$ by Amorphous $TiO_{x}$ Encapsulation

TL;DR

This study demonstrates that amorphous titanium suboxide ($TiO_{x}$) encapsulation significantly improves the doping efficiency and intrinsic mobility of monolayer $MoS_{2}$, achieving record-low contact resistance and high mobility values.

Contribution

It introduces amorphous $TiO_{x}$ as an effective high-$ ext{k}$ dielectric dopant for $MoS_{2}$, with experimental and theoretical analysis of its doping mechanism and mobility enhancement.

Findings

Achieved lowest contact resistance (~180 Ω·μm) for monolayer $MoS_{2}$.

Realized high ON current (240 μA/μm) and field-effect mobility (83 cm²/V·s).

Attained intrinsic mobility up to 102 cm²/V·s at 300 K and 501 cm²/V·s at 77 K.

Abstract

To reduce Schottky-barrier-induced contact and access resistance, and the impact of charged impurity and phonon scattering on mobility in devices based on 2D transition metal dichalcogenides (TMDs), considerable effort has been put into exploring various doping techniques and dielectric engineering using $high-\kappa$ oxides, respectively. The goal of this work is to demonstrate a $high-\kappa$ dielectric that serves as an effective n-type charge transfer dopant on monolayer (ML) molybdenum disulfide ($MoS_{2}$). Utilizing amorphous titanium suboxide (ATO) as the '$high-\kappa$ dopant', we achieved a contact resistance of ~ $180$ ${\Omega}.{\mu}m$ which is the lowest reported value for ML $MoS_{2}$. An ON current as high as $240$ ${\mu}A/{\mu}m$ and field effect mobility as high as $83$ $cm^2/V-s$ were realized using this doping technique. Moreover, intrinsic mobility as high as $102$ $cm^2/V-s$ at $300$ $K$ and $501$ $cm^2/V-s$ at $77$ $K$ were achieved after ATO encapsulation which are among the highest mobility values reported on ML $MoS_{2}$. We also analyzed the doping effect of ATO films on ML $MoS_{2}$, a phenomenon which is absent when stoichiometric $TiO_{2}$ is used, using ab initio density functional theory (DFT) calculations which shows excellent agreement with our experimental findings. Based on the interfacial-oxygen-vacancy mediated doping as seen in the case of $high-\kappa$ ATO - ML $MoS_{2}$, we propose a mechanism for the mobility enhancement effect observed in TMD-based devices after encapsulation in a $high-\kappa$ dielectric environment.