In Situ Studies on the Influence of Surface Symmetry on the Growth of MoSe₂ Monolayer on Sapphire Using Reflectance...

The surface symmetry of the substrate plays an important role in the epitaxial high-quality growth of 2D materials; however, in-depth and in situ studies on these materials during growth are still limited due to the lack of effective in situ monitoring approaches. In this work, taking the growth of MoSe₂ as an example, the distinct growth processes on Al₂O₃ (112¯0) and Al₂O₃ (0001) are revealed by parallel monitoring using in situ reflectance anisotropy spectroscopy (RAS) and differential reflectance spectroscopy (DRS), respectively, highlighting the dominant role of the surface symmetry. In our previous study, we found that the RAS signal of MoSe₂ grown on Al₂O₃ (112¯0) initially increased and decreased ultimately to the magnitude of bare Al₂O₃ (112¯0) when the first layer of MoSe₂ was fully merged, which is herein verified by the complementary DRS measurement that is directly related to the film coverage. Consequently, the changing rate of reflectance anisotropy (RA) intensity at 2.5 eV is well matched with the dynamic changes in differential reflectance (DR) intensity. Moreover, the surface-dominated uniform orientation of MoSe₂ islands at various stages determined by RAS was further investigated by low-energy electron diffraction (LEED) and atomic force microscopy (AFM). By contrast, the RAS signal of MoSe₂ grown on Al₂O₃ (0001) remains at zero during the whole growth, implying that the discontinuous MoSe₂ islands have no preferential orientations. This work demonstrates that the combination of in situ RAS and DRS can provide valuable insights into the growth of unidirectional aligned islands and help optimize the fabrication process for single-crystal transition metal dichalcogenide (TMDC) monolayers.