Nucleation and growth studies of large-area deposited WS2 on flexible substrates

Abstract

Transition metal dichalcogenides (TMDCs) such as tungsten disulfide (WS2) are studied for advanced electronic and optical devices because of their unique and versatile electrical, optical and mechanical properties. For the use of TMDC films in next-generation flexible electronics, large-area bottom-up synthesis on flexible substrates needs to be mastered, understood and controlled. In this study, we performed a detailed study on the nucleation and growth of WS2 layers deposited by metalorganic chemical vapor deposition (MOCVD) on crystalline van-der-Waals material muscovite mica as a model substrate and on the alkali-metal free flexible glass AF 32® eco. The deposition of the WS2 layers was performed using an all nitrogen-coordinated bis-imido-bis-amido tungsten based precursor in combination with elemental sulfur as the co-reactant. On both substrates, crystalline growth of WS2 at a moderate growth temperature of 600 °C was verified by Raman spectroscopy and X-ray diffraction (XRD). However, the growth mode and nucleation density differ significantly. On mica, an initially planar growth of WS2 triangular islands is observed, whereas untreated glass reveals an out-off plane growth. Detailed XRD and Raman analysis show tensile strain in the WS2 films on both substrates, indicating a strong interaction from CVD grown TMDC films with the underlying carrier material. In order to avoid such substrate-semiconductor interaction, a substrate pre-treatment is required. A plasma pre-treatment prior to the deposition leads to a planar growth even on amorphous glass substrates.