Nucleation and growth mechanisms of Al₂O₃ atomic layerdeposition on synthetic polycrystalline MoS₂

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are of great interest for applications in nano-electronic devices. Their incorporation requires the deposition of nm-thin and continuous high-k dielectric layers on the 2D TMDs. Atomic layer deposition (ALD) of high-k dielectric layers is well established on Si surfaces: the importance of a high nucleation density for rapid layer closure is well known and the nucleation mechanisms have been thoroughly investigated. In contrast, the nucleation of ALD on 2D TMD surfaces is less well understood and a quantitative analysis of the deposition process is lacking. Therefore, in this work, we investigate the growth of Al₂O₃ (using Al(CH₃)₃/H₂O ALD) on MoS₂ whereby we attempt to provide a complete insight into the use of several complementary characterization techniques, including X-ray photo-electron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. To reveal the inherent reactivity of MoS₂, we exclude the impact of surface contamination from a transfer process by direct Al₂O₃ deposition on synthetic MoS₂ layers obtained by a high temperature sulfurization process. It is shown that Al₂O₃ ALD on the MoS₂ surface is strongly inhibited at temperatures between 125°C and 300°C, with no growth occurring on MoS₂ crystal basal planes and selective nucleation only at line defects or grain boundaries at MoS₂ top surface. During further deposition, the as-formed Al₂O₃ nano-ribbons grow in both vertical and lateral directions. Eventually, a continuous Al₂O₃ film is obtained by lateral growth over the MoS₂ crystal basal plane, with the point of layer closure determined by the grain size at the MoS₂ top surface and the lateral growth rate. The created Al₂O₃/MoS₂ interface consists mainly of van der Waals interactions. The nucleation is improved by contributions of reversible adsorption on the MoS₂ basal planes by using low deposition temperature in combination with short purge times. While this results in a more two-dimensional growth, additional H and C impurities are incorporated in the Al₂O₃ layers. To conclude, our growth study reveals that the inherent reactivity of the MoS₂ basal plane for ALD is extremely low, and this confirms the need for functionalization methods of the TMD surface to enable ALD nucleation.