Uncovering the effects of metal contacts on monolayer MoS₂

Metal contacts are a key limiter to the electronic performance of two-dimensional (2D) semiconductor devices. Here, we present a comprehensive study of contact interfaces between seven metals (Y, Sc, Ag, Al, Ti, Au, Ni, with work functions from 3.1 to 5.2 eV) and monolayer MoS₂ grown by chemical vapor deposition. We evaporate thin metal films onto MoS₂ and study the interfaces by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and electrical characterization. We uncover that (1) ultrathin oxidized Al dopes MoS₂ n-type (>2 × 10¹² cm⁻²) without degrading its mobility, (2) Ag, Au, and Ni deposition causes varying levels of damage to MoS₂ (e.g. broadening Raman E′ peak from <3 to >6 cm⁻¹), and (3) Ti, Sc, and Y react with MoS₂. Reactive metals must be avoided in contacts to monolayer MoS₂, but control studies reveal the reaction is mostly limited to the top layer of multilayer films. Finally, we find that (4) thin metals do not significantly strain MoS₂, as confirmed by X-ray diffraction. These are important findings for metal contacts to MoS₂ and broadly applicable to many other 2D semiconductors.