Toward barrier free contact to MoSe2/WSe2 heterojunctions using two-dimensional metal electrodes.

In the design of electronic devices based on two-dimensional heterojunctions, the contact between electrodes and different surfaces of two-dimensional heterojunctions may produce different effects. Furthermore, metal-semiconductor contact plays an important role in modern devices. However, due to the Fermi level pinning effect (FLPE), it is difficult to tune the Schottky barrier height between common metals (e.g. Au, Ag, and Cu) and semiconductors. Fortunately, the FLPE becomes weak at the contact between the 2D metal and 2D semiconductor, due to the suppression of metal-induced gap states. Here, we choose monolayer NbS2 as the electrode to be in contact with the MoSe2/WSe2 bilayer. The interfacial properties as well as the stacking dependence are discussed based on the density functional theory, combined with the nonequilibrium Green's functions. Two configurations are considered, i.e. the WSe2/MoSe2/NbS2 and MoSe2/WSe2/NbS2 stacking sequences. Our results show that barrier free contact can be formed in these 2D metal-semiconductor junctions (MSJs). In addition, the transport properties of the proposed devices are sensitive to the stacking sequence. The drain-source current versus bias voltage (I-V) curve exhibits a linear relationship for the WSe2/MoSe2/NbS2 system and its resistance is much lower than the MoSe2/WSe2/NbS2 MSJ. Detailed analysis reveals that the transport properties are governed by the electronic coupling between specific interlayer states. In WSe2/MoSe2/NbS2 configuration, large overlapping states are observed, which facilitate charge transfer and result in good ohmic contact. Our work may provide a theoretical guidance for the designing of next-generation ultrathin and flexible devices.