Understanding ambipolar transport in MoS field effect transistors: the substrate is the key.

2D materials offer a pathway for further scaling of CMOS technology. However, for this to become a reality, both n-MOS and p-MOS should be realized, ideally with the same (standard) material. In the specific case of MoS field effect transistors (FETs), ambipolar transport is seldom reported, primarily due to the phenomenon of Fermi level pinning (FLP). In this study we identify the possible sources of FLP in MoS FETs and resolve them individually. A novel contact transfer technique is used to transfer contacts on top of MoS flake devices that results in a significant increase in the hole branch of the transfer characteristics as compared to conventionally fabricated contacts. We hypothesize that the pinning not only comes from the contact-MoS interface, but also from the MoS-substrate interface. We confirm this by shifting to an hBN substrate which leads to a 10 fold increase in the hole current compared to the SiO substrate. Furthermore, we analyse MoS FETs of different channel thickness on three different substrates, SiO, hBN and AlO, by correlating the p-branch I /I to the position of oxide defect band in these substrates. FLP from the oxide is reduced in the case of AlO which enables us to observe ambipolar transport in a bilayer MoS FET. These results highlight that MoS is indeed an ambipolar material, and the absence of ambipolar transport in MoS FETs is strongly correlated to its dielectric environment and processing conditions.