Recrystallization of MBE-Grown MoS2 Monolayers Induced by Annealing in a Chemical Vapor Deposition Furnace

A systematic study of MoS2 grown by a combination of physical vapor deposition and post-growth annealing treatment has been conducted. Hereby, MoS2 thin films with thicknesses between 1 and 2 layers are first grown on sapphire by molecular beam epitaxy at different growth temperatures and then transferred to S environment inside a tube furnace for an annealing process. Depending on the growth temperature, the as-grown layers are either amorphous or form a crystalline structure composed of closely packed nanometer-size grains. The annealing process leads to recrystallization of these layers significantly increasing the size of the MoS2 crystalline domains to the range of 50-100 nm. While the originally amorphous layer displays rotational domains after annealing, recrystallization of samples grown at high temperatures yields single crystalline layers. All samples display an increase of the crystallite dimension, which is accompanied by the disappearance of the defect-related peaks in the Raman spectra, sharpening of the excitonic signatures in absorption, and strong enhancement of the photoluminescence yield. The results represent a promising way to combine advantages of physical vapor deposition and a post-growth annealing in a chemical vapor deposition furnace toward fabrication of wafer-scale single crystalline transition metal dichalcogenide mono- and multilayer films on non-van der Waals substrates.