P-type Schottky-barrier-free contact to MoS2 via layer-number-assisted interface engineering

Bilayer and few-layer MoS2 show intrinsically higher electronic quality and substantially improved device performance than monolayer, and high-quality MoS2 wafers with controlled layer number have been grown. In addition, MoS2 has different polymorphs such as the semiconducting H phase and semimetallic distorted T (dT) phase, and vertically stacked dT-/H-MoS2 metal-semiconductor junctions (MSJs) have been synthesized. However, the contact mechanism of dT-/H (few layer)-MoS2 MSJs remains to be elucidated, and p-type ohmic contact to MoS2 is difficult to realize due to the high ionization energy of MoS2. In the current work, we reveal a mechanism of two-dimensional (2D) semiconductors (2DSCs) layer-number-assisted metal-semiconductor (SC) interface engineering for Schottky barrier height (SBH) reduction and p-type ohmic contact is achieved based on this mechanism; 2DSCs here mean H (few layer)-MoS2 and metal-SC interfaces are the dT-/H-MoS2 interfaces with increasing 2DSC layers. Specifically speaking, the mechanism is as follows: (1) two competing effects coexist, namely, the interface dipole (AV) at the metal-SC interface and the quasibonding (QB) between all adjacent layers, with A V (QB) increasing (decreasing) SBH; (2) the effect of QB beats A V and hence the overall effect is decreasing SBH at the metal-SC interface; and (3) the SBH reduction effect increases with increasing 2DSC layers. The mechanism of 2DSC layer-number-assisted metal-SC interface engineering should apply also for other 2DSCs with a suitable metal and hence our current work paves an avenue for ohmic contact to few-layer 2DSCs by the accumulated interlayer QBs that widely present in 2DSCs.