▎ 摘　　要

Group-III nitride (III-N) films have numerous applications in LEDs, lasers, and high-power/high-frequency electronic devices because of their direct wide band gap, high breakdown voltage, high saturation velocity of electrons, and high stability. Commercial III-N films are usually heteroepitaxially grown on c-sapphire substrate by metal-organic chemical vapor deposition (MOCVD). However, relatively large mismatches occur in the in-plane lattice and thermal expansion between the III-N films and sapphire substrates, which lead to high stress and high dislocation density in epilayers that reduce the performance of the LED. Moreover, the poor thermal conductivity of sapphire substrate also hinders many applications. Recently, graphene was used as a buffer layer to overcome the mismatch between III-N films and substrates by utilizing van der Waals epitaxy and improving heat dissipation. In this review article, we consider the recent progress in the development of a new type of epitaxial substrate, the so-called "graphene/sapphire substrate" for III-N film growth and LED applications. The growth mechanisms are summarized and future prospects are proposed. The article is divided into three parts. 1. The synthesis of graphene/sapphire substrate. High-quality monolayer graphene is directly synthesized on sapphire substrates (flat substrate and nanopatterned substrate) by metal-catalyst-free CVD method. The method does not depend on the metal catalyst nor involve a complex and highly technical transfer process, and is compatible with the MOCVD and molecular beam epitaxy process. 2. Growth of high-quality III-N films on graphene/sapphire substrates. The nucleation of III-N on graphene can be tuned by the density of defects in the graphene film. N-2 plasma treatment of the graphene/sapphire substrate can increase the nucleation sites for III-N growth by introducing pyrrolic nitrogen doping. Epitaxial lateral overgrowth of the III-N is promoted on the graphene/sapphire substrate owing to the relatively lower diffusion barrier of atoms on graphene. Consequently, the biaxial stress in group-III nitride is significantly decreased while the dislocation density is reduced even without a low-temperature buffer layer. Moreover, vertically-oriented graphene nanowalls can effectively improve the heat dissipation in AlN films. 3. High-performance LEDs on graphene/sapphire substrate. High-quality III-N films obtained on graphene/sapphire substrates enable LED fabrication. The as-fabricated LEDs on graphene/sapphire substrate deliver much higher light output power compared with that on bare sapphire substrate. The as-fabricated LEDs have low turn-on voltage, high output power, and good reliability. Graphene can also be utilized as transfer medium or transparent conductive electrode to boost LED performance.