Direct Atomic Layer Deposition of Ultrathin Aluminum Oxide on Monolayer MoS2 Exfoliated on Gold: The Role of the Substrate

In this paper, the authors demonstrate the atomic layer deposition (ALD) of highly homogeneous and ultrathin (approximate to 3.6 nm) Al2O3 films with very good insulating properties (breakdown field of approximate to 10-12 MV cm(-1)) directly onto monolayer (1L) MoS2 exfoliated on gold. Differently than in the case of 1L MoS2 supported by a common insulating substrate (Al2O3/Si), a better nucleation process of the high-k film is observed on the 1L MoS2/Au system since the ALD early stages. Atomic force microscopy analyses show a approximate to 50% Al2O3 surface coverage just after 10 ALD cycles, its increase to >90% (after 40 cycles), and a uniform approximate to 3.6 nm film (after 80 cycles). The Al2O3 density on bilayer MoS2 is found to be significantly reduced with respect to 1L MoS2/Au, suggesting a role of screened interface charges with the metal substrate on the adsorption of ALD precursors. Finally, Raman and photoluminescence spectroscopy show a p-type doping and tensile strain of 1L MoS2 induced by the Au substrate, providing an insight on the evolution of vibrational and optical properties after the Al2O3 deposition. The direct ALD growth of Al2O3 on large-area 1L MoS2 induced by the Au underlayer can be of wide interest for electronic applications.