Thermal stability of MBE-grown epitaxial MoSe2 and WSe2 thin films*

Layered transition metal dichalcogenides (TMDs) draw much attention, because of its unique optical properties and band structures depending on the layer thicknesses. However, MBE growth of epitaxial films demands information about thermal stability of stoichiometry and related electronic structure for high temperature range. We grow epitaxial MoSe2 and WSe2 ultrathin films by using molecular beam epitaxy (MBE). We characterize stoichiometry of films grown at various growth temperature by using various methods, XPS, EDX, and TOF-MEIS. We further test high temperature stability of electronic structure for those films by utilizing in-situ ellipsometry attached to UHV chamber. We discuss threshold temperatures up to 700\textasciitilde 1000$^{\mathrm{o}}$C, at which electronic phases changes from semiconductor to metal due to selenium deficiency. This information can be useful for potential application of TMDs for fabrication of Van der Waals multilayers and related devices.