Effect of growth temperature on the microstructure and properties of epitaxial MoS₂ monolayers grown by metalorganic chemical vapor deposition

Metalorganic chemical vapor deposition (MOCVD) is a promising technique for wafer-scale synthesis of MoS₂ monolayers for 2D field-effect transistors (2D-FETs) and related devices. Epitaxial growth of MoS₂ on sapphire provides films that are crystallographically well-oriented but typically contain low-angle grain boundaries (e.g., mirror twins), voids, and other defects depending on growth conditions and substrate characteristics. In this study, we investigate microstructure, optical properties, and field-effect characteristics of wafer-scale MoS₂ monolayers grown by MOCVD on c-plane sapphire over a narrow window of growth temperatures (900-1000 °C). The density of low-angle grain boundaries in the MoS₂ monolayer was found to decrease dramatically from 50% areal coverage for films grown at 900 °C to 5% at 1000 °C. This decrease in low-angle grain boundary density is correlated with an increase in the room-temperature photoluminescence intensity of A excitons and a decrease in the full-width-half maximum (FWHM) of the Raman A_1g peak, which are typically indicative of a general reduction in defects in MoS₂. However, the best transport properties (e.g., mean field-effect mobility m_FE= 17.3 cm²/V s) were obtained in MoS₂ monolayers grown at an intermediate temperature of 950 °C. It was found that as the growth temperature increased, small regions bound by high-angle boundaries begin to appear within the monolayer and increase in areal coverage, from ∼2% at 900 °C to ∼5% at 950 °C to ∼10% at 1000 °C. The growth temperature of 950 °C, therefore, provides an intermediate condition where the combined effects of low-angle and high-angle boundaries are minimized. The results of this study provide guidance on MOCVD growth and characterization that can be used to further optimize the performance of MoS₂ 2D-FETs.