Effect of in situ annealing on the structural properties of Bi2Te3 films grown on (0 0 0 1) sapphire

•Comprehensive analysis of the Bi2Te3 restructuring due to an in situ annealing.•Change from polycrystal to single crystal by the restructuring.•The crystallinity and surface roughness were improved due to the annealing.•Preventing Bi2Te3 decomposition by applying Te flux during the annealing.•Utilizing the annealed layer to growth of thick, high quality Bi2Te3films.