Surface Defect Engineering of MoS₂ for Atomic Layer Deposition of TiO₂ Films

In this manuscript, we combine experimental and computational approaches to study the atomic layer deposition (ALD) of dielectrics on MoS2 surfaces for a very common class of ALD precursors, the alkylamines. More specifically, we study the thermal ALD of TiO2 from TDMAT and H2O. Depositions on as-produced chemical vapor deposition MoS2 flakes result in discontinuous films. Surface treatment with mercaptoethanol (ME) does not improve the surface coverage, and DFT calculations show that ME reacts very weakly with the MoS2 surface. However, creation of sulfur vacancies on the MoS2 surface using Ar ion beam irradiation results in much improved surface coverage for films with a nominal thickness of 6 nm, and the calculations show that TDMAT reacts moderately with either single or extended sulfur vacancies. ME also reacts with the vacancies, and defect-rich surfaces treated with ME provide an equally good surface for the nucleation of ALD TiO2 films. The computational studies however reveal that the creation of surface vacancies results in the introduction of gap states that may deteriorate the electronic properties of the stack. Treatment with ME results in the complete removal of the gap states originating from the most commonly found single vacancies and reduces substantially the density of states for double and line vacancies. As a result, we provide a pathway for the deposition of high-quality ALD dielectrics on the MoS2 surfaces, which is required for the successful integration of these 2D materials in functional devices.