Direct atomic layer deposition of ultra-thin $A{l}_2{O}_3$ and $Hf{O}_2$ films on gold-supported monolayer $Mo{S}_2$

Abstract

In this paper, the atomic layer deposition (ALD) of ultra-thin films (<4 nm) of $A{l}_2{O}_3$ and $Hf{O}_2$ on Au-supported monolayer (1L) $Mo{S}_2$ is investigated, providing an insight on the nucleation mechanisms in the early stages of the ALD process. A preliminary multiscale characterization of large area 1L-$Mo{S}_2$ exfoliated on sputter-grown Au/Ni films revealed an almost conformal $Mo{S}_2$ membrane with the Au topography and the occurrence of strain variations at the nanoscale. Ab-initio DFT calculations of $Mo{S}_2$/Au(111) interface showed a significant influence of the Au substrate on the $Mo{S}_2$ energy band structure, whereas small differences were accounted for the adsorption of the $H_{2}O$, TMA and TDMAHf precursors. This suggests a crucial role of nanoscale morphological effects, such as local curvature and strain of the $Mo{S}_2$ membrane, in the enhanced physisorption of the precursors. Therefore, the nucleation and growth of $A{l}_2{O}_3$ and $Hf{O}_2$ films onto 1L-$Mo{S}_2$/Au was investigated, by monitoring the surface coverage as a function of the number (N) of ALD cycles, with N from 10 to 120. At low N values, a slower growth rate of the initially formed nuclei was observed for $Hf{O}_2$, probably due to the bulky nature of the TDMAHf precursor as compared to TMA. On the other hand, the formation of continuous films was obtained in both cases for N>80 ALD cycles, corresponding to 3.6 nm $A{l}_2{O}_3$ and 3.1 nm $Hf{O}_2$. Current mapping by C-AFM showed, for the same applied bias, a uniform insulating behavior of $A{l}_2{O}_3$ and the occurrence of few localized breakdown spots in the case of $Hf{O}_2$, associated to less compact films regions. Finally, an increase of the 1L-$Mo{S}_2$ tensile strain was observed by Raman mapping after encapsulation with both high-k films, accompanied by a reduction in the PL intensity.

26 pages, 9 figures