ALL-2D SEMICONDUCTOR/METAL CONTACTS

摘要

First-principles calculations are used to explore the geometry, bonding, and electronic properties of MoS_2/Ti_2C and MoS_2/Ti_2CY_2 (Y = F and OH) semiconductor/ metal contacts. Strong chemical bonds are formed at the MoS_2/Ti_2C interface and result in additional states around the Fermi level, which extend over the three atomic layers of MoS_2 and induce a metallic character. The interaction in MoS_2/Ti_2CY_2, on the other hand, is weak and not sensitive to the geometry, and the semiconducting nature of MoS_2 is preserved. The energy level alignment implies a weak and strong n-type doping of MoS_2 in contact with Ti_2CF_2 and Ti_2C(OH)_2, respectively, with corresponding Schottky barrier heights of 0.85 and 0.26 eV. We show that the MoS_2/ Ti_2CF_2 contact is close to the Schottky limit, whereas at the MoS_2/Ti_2C(OH)_2 contact we obtain a strong dipole due to charge rearrangement. The weak interaction between MoS_2 and Ti_2CY_2 can be interpreted as physisorp-tion. As a consequence, the electronic properties of the Ti_2CY_2 subsystem are hardly changed, in particular the characteristic metallicity. The conduction and valence bands largely maintain their characters, with the CBM and VBM still being located at the K point, though the interaction enhances the band gaps by 0.13 and 0.16 eV, respectively. These increments mainly can be ascribed to the lattice compression of 1.3%. Taking the vacuum level as reference value, we can align the energy levels of the two hybrid systems to get detailed insight into the CBM and VBM shifts in MoS_2 upon physi-sorption. The lattice compression induces shifts of the VBM and CBM by 0.15 and 0.29 eV, respectively. We find the Fermi level in MoS_2/Ti_2CF2 only 0.06 eV above the midgap, (CBM+VBM)/2, suggesting weakly n-type doped MoS_2, while it is much closer to the CBM in MoS_2/Ti_2C(OH)_2, indicating stronger doping. This gives rise to a flexible approach to realize n-type doping of MoS_2 by 2D material contacts. The investigated interfaces therefore are well suited for application in all-2D devices.