Modeling the coverage of MoS₂ and WS₂ thin films using in-situ spectroscopic ellipsometry

In-situ spectroscopic ellipsometry (SE) was used to analyze monolayer and few-layer samples of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. MoS₂ and WS₂ film growth times ranged from 5 – 40 min to achieve a range of film coalescence and surface coverage from less than a monolayer to considerable bilayer and additional layers. Post-growth measurements using atomic force microscopy, photoluminescence and Raman spectroscopy were used to assess the evolution of surface morphology and film properties with growth time to determine the areal coverage of monolayer, bilayer, multilayer and void regions. The room temperature SE spectra were modeled using an effective medium approximation to obtain the monolayer and bilayer dielectric functions of MoS₂ and WS₂. Using these distinct dielectric functions, changes in ellipsometry parameters associated with partial to full coverage of monolayers and bilayers were simulated. Such simulations will enable improved control of the layer-by-layer deposition of MoS₂ and WS₂ by monitoring in-situ spectroscopic ellipsometry.