Bonding and electronics of the ${\mathrm{MoTe}}_2/\mathrm{Ge}$ interface under strain

Understanding the interface formation of a conventional semiconductor with a monolayer of transition-metal dichalcogenides provides a necessary platform for the anticipated applications of dichalcogenides in electronics and optoelectronics. We report here, based on the density functional theory, that under in-plane tensile strain, a 2H semiconducting phase of the molybdenum ditelluride $\left({\mathrm{MoTe}}_2\right)$ monolayer undergoes a semiconductor-to-metal transition and in this form bonds covalently to bilayers of Ge stacked in the [111] crystal direction. This gives rise to the stable bonding configuration of the ${\mathrm{MoTe}}_2$/Ge interface with the $\pm \mathrm{K}$ valley metallic, electronic interface states exclusively of a Mo $4d$ character. The atomically sharp Mo layer represents therefore an electrically active (conductive) subsurface $\delta$-like two-dimensional profile that can exhibit a valley-Hall effect. Such system can develop into a key element of advanced semiconductor technology or a novel device concept.

Figure 1

Electronic band structures of (a) the isolated Ge(111) seven bilayers film (solid lines) with the bulk Ge band structure projected on the [111] direction (shaded area), (b) the isolated unstrained ${\mathrm{MoTe}}_2$ monolayer, (c) the isolated strained ${\mathrm{MoTe}}_2$ monolayer, (d) the ${\mathrm{MoTe}}_2$/Ge interface in configuration ${\mathrm{H}}_3$, and (e) the ${\mathrm{MoTe}}_2$/Ge interface in configuration ${\mathrm{T}}_1$. A schematic of SBZ with high symmetry points indicated is shown next to (a). The shaded area in (d) and (e) represents the bulk Ge band structure projected on the [111] direction.

Figure 2

Schematics of atomic structures (bottom and side views) of ${\mathrm{MoTe}}_2$/Ge(111) interfaces in configuration ${\mathrm{H}}_3,{\mathrm{T}}_1$, and ${\mathrm{T}}_4$. The surface unit cell, equivalent to that for Ge(111)-$1\times 1$, is shown in configuration ${\mathrm{T}}_1$. The bottom view presents ${\mathrm{MoTe}}_2$ and the first bilayer of the Ge film.

Figure 3

Relative energetics of the ${\mathrm{MoTe}}_2$/Ge interfaces as a function of a number of bilayers $\left(n\right)$ in the Ge film. The reference energy is that of configuration ${\mathrm{T}}_1$.

Figure 4

Partial (pseudo-)charge densities $\left({\left|\psi \right|}^2\right)$ calculated for the states at point $K$ for bands (a) 6, (b) 7, and (c) 8 in Figs. 1 and 1. Isosurface value: $0.0025e$/a.u.${}^3$.