A simple approach to analyze layer-dependent optical properties of few-layer transition metal dichalcogenide thin films

Abstract

Intense interest in transition metal dichalcogenides (TMDs) is driven, among other things, by their intriguing layer-dependent optical properties down to the monolayer and strong excitonic effects. Experimental determination of optical constants such as the complex refractive index and dielectric function of large-area TMDs thin films is prone to large uncertainties at the few-layer level using a single measurement technique such as spectroscopic ellipsometry (SE). In this work, we demonstrate an approach to accurately determine the optical constants by combining transmission spectroscopy with SE. Using MoS2 as an example, the prototypical TMD material, we demonstrate our approach on transparent (Quartz, Al2O3) and absorbing (e.g. Si/SiO2) substrates. We find that pre-characterized optical properties from transmission spectroscopy significantly improve the quantitative accuracy of optical constants obtained using SE on both transparent and absorbing substrates. We show that the extracted optical constants from samples deposited on transparent substrates are highly reliable in obtaining layer-dependent optical properties of TMDs while data on absorbing Si/SiO2 substrate suffer from strong substrate contribution. Overall, we strongly conclude that SE must be combined with transmission spectroscopy to obtain optical constants with high quantitative accuracy. Our approach, though demonstrated on few-layer films, will be applicable to monolayer and bilayer TMDs.