Monolayer-thin WS₂ with (0002) texture grows by chemical vapor deposition (CVD) from gas-phase precursors WF₆ and H₂S at a deposition temperature of 450 °C on 300 mm Si wafers covered with an amorphous Al₂O₃ starting surface. We investigate the growth and nucleation mechanism during the CVD process by analyzing the morphology of the WS₂ crystals. The CVD process consists of two distinct growth regimes. During (i) the initial growth regime, a fast and self-limiting reaction of the CVD precursors with the Al₂O₃ starting surface forms predominantly monolayer-thin WS₂ crystals and AlF₃ crystals that completely cover the starting surface. During (ii) the steady-state growth regime, a much slower, anisotropic reaction on the bottom, first WS₂ layer proceeds with the next WS₂ layer growing preferentially in the lateral dimensions. We propose that the precursor adsorption reaction rate strongly diminishes when the precursors have no more access to the Al₂O₃ surface as soon as the WS₂ layer completely covers the Al₂O₃ surface and that the WS₂ crystal basal planes and AlF₃ crystals have a low reactivity for WF₆ adsorption at 450 °C. Nonetheless, a second layer of WS₂ starts to form before the first WS₂ layer completely covers the starting surface, albeit the surface coverage of the second layer is low (<20%, after 25 min of CVD reaction). During the steady-state growth regime, predominantly the WS₂ crystals in the second monolayer continue to grow in lateral dimensions up to ∼40 nm. These crystals reach larger lateral dimensions compared to the crystals in the bottom, first layer due to low reactivity for WF₆ adsorption on the WS₂ basal plane compared to Al₂O₃. Presumably, they grow laterally by precursor species that adsorb on and diffuse across the WS₂ surface, before being incorporated at the more reactive edges of the WS₂ crystals in the second layer. Such a process proceeds slowly with only up to 40% surface coverage of the second WS₂ layer after 150 min of CVD reaction. The CVD reaction is mediated by the starting surface: WF₆ precursor preferentially adsorbs on Al₂O₃, whereas adsorption is not observed on SiO₂. Nevertheless, WS₂ grows on SiO₂ in close proximity to Al₂O₃ in 90 nm pitch Al₂O₃/SiO₂ line patterns. Hence, functionalization of the starting surface (e.g., SiO₂ with Al₂O₃) can provide opportunities to grow monolayer-thin WS₂ crystals at predetermined locations by selective, lateral growth with tunable crystal size, even at low deposition temperatures.

#	Concept	H-Index	Publications	Citations
1	Engineering	1	1	65
2	Applied Physics	1	1	65
3	Electrical Engineering	1	1	65
4	Physics	1	1	65
5	Microelectronics	1	1	65

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