Using Spectroscopy of Many Body Physics to Map Spatial Variations in Strain, Electron-Density and Point-Defect Density in Wafer-Scale, CVD Grown, 2D Transition Metal Dichalcogenides

Two dimensional (2D) transition metal dichalcogenides (TMD) are promising channel materials for sub-10 nm gate-length Field Effect Transistors (FET) due to their high carrier mobility and gate control at atomic thicknesses. However, spatial variations in properties of wafer scale 2D TMD films can adversely affect channel mobilities and threshold voltages in fabricated devices. We show that exciton and trion (exciton-polaron) energies in TMDs can serve as natural and sensitive probes of disorder in TMDs without the need for fabricating devices. We show that optical spectroscopy of exciton-trion states and defects at room temperature in wafer-scale monolayer MoS₂ grown by Chemical Vapor Deposition (CVD) can be us­ed to map variations in strain, electron-density, dielectric constant and point-defect densities. The large optical oscillator strengths of exciton-trion states in TMDs allow us to use low optical pump powers (1.5 μW/μm²), ensuring that TMD films are not altered or damaged during measurements. We compare samples grown using different CVD techniques and on different substrates. We find that whereas strain and carrier density can vary spatially by as much as 1% and 10¹³ 1/cm², respectively, in most high quality samples the corresponding variances are around 0.2% and 2x10¹² 1/cm². The lack of spatial correlations between strain and carrier density variations lead us to conclude that charged impurities are primarily responsible for carrier density variations. Our measured variations in the carrier densities correspond well to variations in threshold voltages of fabricated FETs. TMD films grown under metal rich conditions exhibit bare and/or decorated chalcogen vacancies which quench exciton-trion PL. Our work elucidates correlations between exciton-trion energies and material disorder, enabling optical spectroscopy to be a fabrication free, high throughput tool for measuring disorder in wafer-scale 2D TMDs.