Schottky-barrier formation on a covalent semiconductor without Fermi-level pinning: The metal-MoS2(0001) interface

Chemical interaction and Schottky-barrier formation at the metal-${\mathrm{MoS}}_2$ interface were studied by evaporating metals (Ag, Al, Au, Co, Fe, In, Mn, Pd, Rh, Ti, and V) onto the (0001) basal-plane surface of cleaved molybdenite, and then analyzing the interface with x-ray photoelectron spectroscopy (XPS). Except for Mn, negligible changes were revealed in the Mo (3$d_{5/2}$) and S (2$p_{3/2}$) peak shapes, or widths, after deposition. The shifts in the binding energies did not correlate with the electron configuration of the metal but rather with the metal electronegativity, and are interpreted in terms of band bending at the metal-semiconductor interface, rather than chemical reaction. Plots of both Mo and S binding energies versus metal electronegativity yield approximately linear curves with nonzero (positive) slopes, which provide an average ‘‘index of interface behavior’’ of S’=1.28±0.22. This value is considerably higher than for other covalent semiconductors, which exhibit S’<0.3 due to Fermi-level pinning. The anomalous behavior of ${\mathrm{MoS}}_2$ results from the extreme inertness of the basal-plane surface and the stability of the layered crystal lattice of ${\mathrm{MoS}}_2$. The absence of chemical interaction at the interface causes the formation of a Schottky barrier exhibiting behavior that may approach the Schottky limit. This behavior for ${\mathrm{MoS}}_2$(0001) is compared with that of other semiconductors, and is discussed in terms of their ionicity, reactivity, and dielectric response. Metals (Ag, Au, Co, Cu, Fe, Pd, Ti, and V) were also deposited onto ${\mathrm{MoS}}_2$(0001) surfaces that were bombarded with 10-keV ${\mathrm{Ar}}^{+}$ ions. The Fermi level was not strongly pinned, even though defect densities as high as ∼3×${10}^{14}$ ${\mathrm{cm}}^{\mathrm{-}2}$ were produced in the surface region.