▎ 摘　　要

In the present work, improved crystal quality of GaN on a graphene-covered sapphire substrate was achieved compared to GaN grown on a bare sapphire substrate, and the growth mechanism of GaN quasi-van der Waals epitaxy with an AlN nucleation layer was clarified using metal-organic chemical vapor deposition. The in situ N-doping of graphene by an NH3 source during AlN growth was responsible for AlN nucleation. The first-principles calculation indicated that N atoms doped initially at the step edges of graphene and subsequently at its center and AlN nuclei followed the same sequence, which is consistent with experimental observations of nucleation. The lower migration barrier of Al atoms (0.07eV) on graphene created larger AlN nuclei compared to that on bare sapphire (0.21eV), leading to improved quality of the upper GaN epilayer with lower defect density. This work offers guidance for precisely controlling the nucleation morphology and density of GaN base materials and thus realizing high-quality epitaxial materials and related high-performance devices by quasi-van der Waals epitaxy.