Structure and electronic bandgap tunability of m-plane GaN multilayers

Two-dimensional (2D) gallium nitride (GaN) has attracted a lot of attention due to its promising applications in photoelectric nano-devices. Most previous research studies have focused on polar c-plane 2D structures. Here, by employing first principles calculations, we systematically investigate the structural and electronic properties of non-polar m-plane GaN with different numbers of atomic layers. The results show a layer-dependent structure transition and electronic band variation for m-plane GaN. It is found that the monolayer keeps a planar hexagonal structure due to sp2 hybridization, whereas the multilayers are formed by stacking of buckled hexagonal monolayers with unsaturated coordination number at the surface sublayer and bulk-like inner layers. These discrepancies in the structure further induce an indirect to direct transition of the band gap type when the layer number reaches twelve. By carefully examining the relationship between the structure and electronic bandgap, we find that the indirect bandgap comes from the unsaturated surface with a planar like structure. On surface modification, saturation of the surface dangling bonds results in an indirect to direct band gap transition.