Metal surface oxidation as a general route for p -type ohmic contacts to MoS2

The potential applications of two-dimensional (2D) molybdenum disulfide ($\mathrm{Mo}{\mathrm{S}}_{2}$) in ultrathin digital logic circuits have stimulated intensive research. When 2D $\mathrm{Mo}{\mathrm{S}}_{2}$ connects to external circuits to form field-effect transistors based on complementary metal-oxide-semiconductor technology, transport of both electrons ($n$ type) and holes ($p$ type) is necessary. However, the $p$-type ohmic contact to $\mathrm{Mo}{\mathrm{S}}_{2}$ remains challenging. In the current work, based on first-principles calculations, we propose metal surface oxidation, serving as a general route for the regulation of the Schottky barrier from $n$ to $p$ type and the suppression of Fermi-level pinning simultaneously at metal-$\mathrm{Mo}{\mathrm{S}}_{2}$ interfaces. By introducing an ultrathin insulating layer, the oxidized metal surface layer has the following four advantages: (1) Surface oxidation greatly increases the work function of the metal surface, which is crucial for the realization of a $p$-type contact. (2) Surface oxidation passivates the metal surface and hence suppresses the metal-induced gap states in $\mathrm{Mo}{\mathrm{S}}_{2}$. (3) Due to the out of plane ${p}_{z}$-orbital coupling between oxygen and sulfur, significant quasibonding-induced gap states appear above the valence-band maximum of $\mathrm{Mo}{\mathrm{S}}_{2}$, which is beneficial to the reduction of $p$-type Schottky barrier height. (4) Surface oxidation is easily achieved for common transitional precious metal substrates, which ensures that it is a viable way to achieve $p$-type ohmic contacts in metal-$\mathrm{Mo}{\mathrm{S}}_{2}$ heterojunctions. Hence, this study suggests that metal surface oxidation is a general route to $p$-type ohmic contacts for $\mathrm{Mo}{\mathrm{S}}_{2}$ and other 2D semiconductors with out of plane orbitals in the valence band.