Atomic vs. sub-atomic layer deposition: impact of growth rate on the optical and structural properties of MoS2 and WS2

MoS2 and WS2 mono- and multilayers were grown on SiO2 /Si substrates. Growth by atomic layer deposition at fast growth rates is compared to sub-atomic layer deposition, which is a slow growth rate process with only partial precursor surface coverage per cycle. A Raman spectroscopic analysis of the intensity and frequency difference of the modes reveals different stages of growth from partial to full surface layer coverage followed by layer-by-layer formation. The initial layer thickness and structural quality strongly depends on the growth rate and monolayers only form using sub-atomic layer deposition. Optical activity is demonstrated by photoluminescence characterisation which shows typical excitonic emission from MoS2 and WS2 monolayers. A chemical analysis confirming the stoichiometry of MoS2 is performed by X-ray photoelectron spectroscopy. The surface morphology of layers grown with different growth rates is studied by atomic force microscopy. Plan-view transmission electron microscopy analysis of MoS2 directly grown on freestanding graphene reveals the local crystalline quality of the layers, in agreement with Raman and photoluminescence results.