In-plane and out-of-plane domain orientation dispersions in 1 to 3 monolayers epitaxial WS₂ and MoS₂ films on GaN(0001) film/sapphire(0001)

Transition-metal dichalcogenides and their heterostructures have attractive potential applications in electronics and optoelectronics. Wafer scale 1 to 3 monolayers WS₂ and MoS₂ ultrathin films on GaN/sapphire substrates were grown by metal organic chemical vapor deposition. Azimuthal reflection high-energy electron diffraction (ARHEED) was used to characterize the long-range order of these TMDC ultrathin films. The RHEED patterns of WS₂ and MoS₂ show stripes and arcs but the MoS₂ on GaN shows sharp spots in addition to stripes and arcs. The 2D map constructed from ARHEED patterns shows that WS₂ is epitaxial and has an in-plane domain orientation dispersion. For the MoS₂ on GaN/sapphire substrate, the 2D map shows concentric continuous rings for each diffraction order of MoS₂ and GaN indicating that the in-plane MoS₂ domain orientation and GaN nanocrystals are random. The out-of-plane orientation dispersion of MoS₂ on the GaN substrate is larger than that of WS₂ on the GaN substrate. The observations of stripes, arcs, and spots from RHEED patterns and the 2D maps reveal the deviation of ultrathin epitaxial films from its perfect epitaxy, especially the TMDC domain orientation dispersion over a large area. These rich findings from 2D maps broaden the application of ARHEED in more than one monolayer thick 2D materials.