The effect of growth temperature and metal-to-chalcogen on the growth of Wse_2 by molecular beam epitaxy

摘要

To further the present understanding of growth conditions on the quality of transition metal dichalcogenide(TMDC) thin films grown by molecular beam epitaxy (MBE), we study the effect of growth temperature and chalcogento-metal flux ratio on the chemical composition and surface morphology of synthesized WSe_2 thin films. In-situ X-rayphotoelectron spectroscopy (XPS) is performed to analyze the intrinsic chemical composition of the grown material priorto atmospheric exposure and ex-situ atomic force microscopy (AFM) is employed to study the surface morphology ofgrown, sub-monolayer films. We find that both low and high growth temperature ranges can be detrimental to the chemicalhomogeneity of the grown material and that these results are echoed in the resulting grain morphology. Growing at 375 °Cresulted in the formation of metastable 1T’-WSe_2 alongside the thermodynamically stable 2H phase. Thin films grown at750 °C resulted in the formation of highly Se deficient material. An intermediate growth temperature of 565 °C producedthe most chemically homogeneous films above a critical chalcogen to metal flux ratio of 3250:1. Density functional theorycalculations are used to rationalize the insights gained from the measured XPS data. Especially, the influence of Se-vacantWse_(2-x) monolayers is explored and its impact on the coordination environment around the Se-atoms is used to interpretthe measured XPS data.