Role of defects in enhanced Fermi level pinning at interfaces between metals and transition metal dichalcogenides

Density functional theory calculations are performed to explore the nature of the contact between metal electrodes and defected monolayer ${\mathrm{MoSe}}_2$. Partial Fermi level pinning is observed at perfect ${\mathrm{MoSe}}_2$/metal interfaces. Both As- and Br-substituted ${\mathrm{MoSe}}_2$ will induce extra bands in valence band and conduction band, respectively, which exerts influence on the Schottky barrier height. An enhanced partial Fermi level pinning occurs when As- and Br-substituted ${\mathrm{MoSe}}_2$ make contacts with metal electrodes. Se vacancy in the ${\mathrm{MoSe}}_2$ layer can induce a large amount of interfacial states in the band gap of the ${\mathrm{MoSe}}_2$ layer. As a result, nearly complete Fermi level pinning is observed in Se-vacancy ${\mathrm{MoSe}}_2$/metal contacts. Our work offers insight into the Fermi level pinning at the interfaces between two-dimensional materials and metal electrodes, which is important for the applications of two-dimensional materials in nanoelectronic devices with good performance.

Figure 1

The crystal structures of perfect ${\mathrm{MoSe}}_2$/Au and defect ${\mathrm{MoSe}}_2$/Au from (a) top view and (b) side view. The dark violet balls denote the dopants or vacancies in the ${\mathrm{MoSe}}_2$ layer. (e) The integrated charge density difference of perfect-${\mathrm{MoSe}}_2$/Au and ${\mathrm{V}}_{\text{Se}}-{\mathrm{MoSe}}_2$/Au interfaces.

Figure 2

Band structures of (a) perfect-${\mathrm{MoSe}}_2$, (b) ${\mathrm{As}}_{\text{Se}}-{\mathrm{MoSe}}_2$, (c) ${\mathrm{Br}}_{\text{Se}}-{\mathrm{MoSe}}_2$, and (d) ${\mathrm{V}}_{\text{Se}}-{\mathrm{MoSe}}_2$. Some important impurity bands and defect bands are plotted by red lines. The Fermi level is taken as reference.

Figure 3

(a) The total density of states (TDOS) of ${\mathrm{MoSe}}_2$ monolayer without dopants or defects. (b) The TDOS difference between perfect-${\mathrm{MoSe}}_2$ and corresponding defect ${\mathrm{MoSe}}_2$ monolayers. The Fermi level is taken as reference.

Figure 4

Projected band structures of (a) perfect-${\mathrm{MoSe}}_2$/Au, (b) ${\mathrm{As}}_{\text{Se}}-{\mathrm{MoSe}}_2$/Au, (c) ${\mathrm{Br}}_{\text{Se}}-{\mathrm{MoSe}}_2$/Au, and (d) ${\mathrm{V}}_{\text{Se}}-{\mathrm{MoSe}}_2$/Au. Bands dominated by corresponding ${\mathrm{MoSe}}_2$ layer and Au electrode are plotted by red and gray dots, respectively. The Fermi level is taken as reference.

Figure 5

(a) Variation of electron and hole Schottky barrier height as functions of the work function of metal electrodes. The valence band maximum of ${\mathrm{MoSe}}_2$ is set to zero. (b) Binding energies of all the contacts employed in this work.