Controlling the Schottky barrier at ${\mathrm{MoS}}_2$/metal contacts by inserting a BN monolayer

Making a metal contact to the two-dimensional semiconductor ${\mathrm{MoS}}_2$ without creating a Schottky barrier is a challenge. Using density functional calculations we show that, although the Schottky barrier for electrons obeys the Schottky-Mott rule for high work function $(\gtrsim 4.7$ eV) metals, the Fermi level is pinned at 0.1–0.3 eV below the conduction band edge of ${\mathrm{MoS}}_2$ for low work function metals, due to the metal-${\mathrm{MoS}}_2$ interaction. Inserting a boron nitride (BN) monolayer between the metal and the ${\mathrm{MoS}}_2$ disrupts this interaction, and restores the ${\mathrm{MoS}}_2$ electronic structure. Moreover, a BN layer decreases the metal work function of Co and Ni by $\sim 2$ eV, and enables a lineup of the Fermi level with the ${\mathrm{MoS}}_2$ conduction band. Surface modification by adsorbing a single BN layer is a practical method to attain vanishing Schottky barrier heights.

Figure 1

(a) $\sqrt{3}\times \sqrt{3}R{30}^{\circ }$ ${\text{MoS}}_2$ on top of $2\times 2$ Au(111). The black rhombus indicates the surface supercell. (b) $\sqrt{13}\times \sqrt{13}R13.9^{\circ }$ ${\text{MoS}}_2$ on top of $4\times 4$ Au(111). (c), (d) Top and side view of the ${\text{MoS}}_2/h$-BN/Co(111) structure.

Figure 2

(a) The potential step $\Delta V$ at the ${\text{MoS}}_2$/metal interface vs $W_{\mathrm{M}}$, the work function of the clean metal surface. (b) The Schottky barrier height (SBH) for electrons ${\Phi }_{\text{n}}$. The blue and green dashed lines in (a) and (b) indicate the Schottky-Mott rule and Fermi level pinning, respectively. Inset: Schematic energy diagram of the interface, with $\chi$ the electron affinity of ${\text{MoS}}_2$.

Figure 3

(a) Band structures of the ${\text{MoS}}_2$/Mg(0001) interfaces at the equilibrium distance $d_{\mathrm{eq}}=2.2$ Å and (b) at $d=5$ Å. The red color measures a projection of the wave function on the ${\text{MoS}}_2$ orbitals. The Fermi level is set at zero energy. (c), (d) The electron difference density $\Delta n\left(z\right)$, corresponding to the interface distances of (a) and (b), respectively. (e) The total density of states (DOS) (red) corresponding to (a) ${\mathrm{DOS}}_{\mathrm{ads}}/\mathrm{Mg}$, (blue) of freestanding ${\text{MoS}}_2$ ${\mathrm{DOS}}_{\mathrm{ads}}$, and (green) the difference $\Delta \mathrm{DOS}={\mathrm{DOS}}_{\mathrm{ads}}/\mathrm{Mg}-{\mathrm{DOS}}_{\mathrm{ads}}-{\mathrm{DOS}}_{\mathrm{Mg}}$. The orange shading indicates the position of the gap states, created by the interaction at the interface.

Figure 4

$\Delta V$ vs $W_{\mathrm{M}}$. Inset: ${\Phi }_{\text{n}}$ vs $W_{\mathrm{M}}$. The blue and green dashed lines indicate the Schottky-Mott rule and Fermi level pinning, as in Fig. 2.

Figure 5

(a) Band structures of the ${\text{MoS}}_2$/Co(111) and (b) the ${\text{MoS}}_2/h$-BN/Co(111) interfaces. The red color measures a projection of the wave function on the ${\text{MoS}}_2$ orbitals. The Fermi level is set at zero energy. (c), (d) The corresponding electron difference density $\Delta n\left(z\right)$. The orange line indicates the position of the $h$-BN layer.