1T phase as an efficient hole injection layer to TMDs transistors: A universal approach to achieve p-type contacts

Recently, the fabricated MoS₂ field effect transistors (FETs) with 1T-MoS₂ electrodes exhibit excellent performance with rather low contact resistance, as compared with those with metals deposited directly on 2H-MoS₂ (Kappera et al 2014 Nat. Mater. 13 1128), but the reason for that remains elusive. By means of density functional theory calculations, we investigated the carrier injection at the 1T/2H MoS₂ interface and found that although the Schottky barrier height (SBH) values of 1T/2H MoS₂ interfaces can be tuned by controlling the stacking patterns, the p-type SBH values of 1T/2H MoS₂ interfaces with different stackings are lower than their corresponding n-type SBH values, which demonstrated that the metallic 1T phase can be used as an efficient hole injection layer for 2H-MoS_2. In addition, as compared to the n-type Au/MoS₂ and Pd/MoS₂ contacts, the p-type SBH values of 1T/2H MoS₂ interfaces are much lower, which stem from the efficient hole injection between 1T-MoS₂ and 2H-MoS₂. This can explain the low contact resistance in the MoS₂ FETs with 1T-MoS₂ electrodes. Notably, the SBH values can be effectively modulated by an external electric field, and a significantly low p-type SBH value can be achieved under an appropriate electric field. We also demonstrated that this approach is also valid for WS₂, WSe₂ and MoSe₂ systems, which indicates that the method can most likely be extended to other TMDs, and thus may open new promising avenues of contact engineering in these materials.