Atomic Layer Etching Mechanism of MoS₂ for Nanodevices

Among the layered transition metal dichalcogenides (TMDs) that can form stable two-dimensional crystal structures, molybdenum disulfide (MoS₂) has been intensively investigated because of its unique properties in various electronic and optoelectronic applications with different band gap energies from 1.29 to 1.9 eV as the number of layers decreases. To control the MoS₂ layers, atomic layer etching (ALE) (which is a cyclic etching consisting of a radical-adsorption step such as Cl adsorption and a reacted-compound-desorption step via a low-energy Ar⁺-ion exposure) can be a highly effective technique to avoid inducing damage and contamination that occur during the reactive steps. Whereas graphene is composed of one-atom-thick layers, MoS₂ is composed of three-atom-thick S_(top)-Mo_(mid)-S_(bottom) layers; therefore, the ALE mechanisms of the two structures are significantly different. In this study, for MoS₂ ALE, the Cl radical is used as the adsorption species and a low-energy Ar⁺ ion is used as the desorption species. A MoS₂ ALE mechanism (by which the S_(top), Mo_(mid), and S_(bottom) atoms are sequentially removed from the MoS₂ crystal structure due to the trapped Cl atoms between the S_(top) layer and the Mo_(mid) layer) is reported according to the results of an experiment and a simulation. In addition, the ALE technique shows that a monolayer MoS₂ field effect transistor (FET) fabricated after one cycle of ALE is undamaged and exhibits electrical characteristics similar to those of a pristine monolayer MoS₂ FET. This technique is also applicable to all layered TMD materials, such as tungsten disulfide (WS₂), molybdenum diselenide (MoSe₂), and tungsten diselenide (WSe₂).