Origin of n-type conductivity of monolayer MoS2

Monolayer ${\mathrm{MoS}}_{2}$ is a promising two-dimensional material for electronic and optoelectronic devices. As-grown ${\mathrm{MoS}}_{2}$ is an $\mathit{n}$-type semiconductor, however, the origin of this unintentional doping is still not clear. Here, using hybrid density functional theory, we carried out an extensive study of the often observed native point defects, i.e., ${V}_{\text{S}}$, ${V}_{\text{Mo}}$, ${V}_{\text{S2}}$, ${V}_{\text{MoS3}}$, ${V}_{\text{MoS6}}$, ${\mathrm{Mo}}_{\text{S2}}$, and $\mathrm{S}{2}_{\text{Mo}}$, and found that none of them cause $\mathit{n}$-type doping. Specifically, the S vacancy (${V}_{\text{S}}$), which has been widely attributed to $\mathit{n}$-type conductivity, turns out to be an electron compensating center. We report that hydrogen, which is almost always present in the growth environments, is most stable in its interstitial (${\mathrm{H}}_{\text{i}}$) and H-S adatom forms in ${\mathrm{MoS}}_{2}$ and acts as a shallow donor, provided the sample is grown under S-rich condition. Furthermore, they have high migration barriers (in excess of 1 eV), which would ensure their stability even at higher temperatures, and hence lead to $\mathit{n}$-type conductivity.