A magnesium wetting layer was used to modify the surface structure of Mg Al ₂ O ₄ (111) substrate to achieve growth of high-quality ZnO film by radio frequency plasma-assisted molecular beam epitaxy. It is found that this magnesium layer plays a crucial role in 30° rotation domain elimination, defect density reduction, and polarity control of ZnO film, as demonstrated by in situ reflection high-energy electron diffraction and ex situ transmission electron microscopy. Atomic force microscopy observation shows smooth ZnO surfaces with clearly resolved atomic steps of the films.