Hyperspectral Line Imaging Analysis of Defects in Atomically Thin Semiconductors

Transition Metal Dichalcogenides (TMDs) are van der Waals 2D semiconductors with exotic physical properties originating from its 2D nature. Especially, monolayer TMDs have a direct bandgap which makes them promising for optoelectronic applications. For practical applications of these materials, several challenges need to be addressed: uniform and large-scale growth, high-yield fabrication of device structures, and large-scale characterization of the materials. The Metal Organic Chemical Vapor Deposition (MOCVD) method has been used for wafer-scale growth of TMDs, and researchers have demonstrated the device applications of TMDs such as classical logic gates, neuromorphic devices, gas sensors etc. However, syntheized TMD monolayers are often exhibit spatial heterogeneity such as non-uniform photoluminescence (PL), carrier mobility, and hysteresis, primarily due to defect distribution. Therefore, characterizing these spatially distributed defects is crucial for the practical application of TMDs.<br/>In our presentation, we investigate the defect formation-related variation of optical properties in monolayer WS2 flakes. Using MOCVD, we achieved stoichiometry-dependent growth of WS2 monolayer flakes by adjusting the sulfur precursor injection without altering other growth conditions. We observed that WS2 flakes exhibit uniform PL intensity with an increasing the sulfur/tungsten precursor ratio. By utilizing Hyperspectral Line Imaging (HSLI), we statistically analyzed the spectral properties of WS2 monolayer flakes. Similar to PL intensity, W-rich flakes exhibit the most inhomogeneous distribution of exciton peak positions compared to S-rich flakes. We further examined the optical response after chemical treatment, observing contrasting behavior in W-rich and S-rich flakes, which we attributed to different types of defects. By comparing with defect formation energy calculations using density functional theory, we identified sulfur vacancy defects (V_s) as the dominant defect in W-rich flakes and chalcogen antisite defects (S_w) as the most prevalent defect in S-rich flakes. To demonstrate the large-scale applicability of HSLI, we conducted similar analysis on WS2 monolayer films and successfully characterized W-rich and S-rich samples on a sub-centimeter scale.