Role of AlN intermediate layer in the morphological evolution of GaN nanorods grown on c-plane sapphire

Abstract

The difference in the evolution of dislocation mediated low-dimensional GaN nanostructures on differently pre-nitrided c-plane sapphire surfaces is demonstrated. We show that GaN growth on c-plane sapphire, after forming an AlN layer by exposure to N-2* plasma, leads to the formation of nano-petal-like morphologies of the GaN nanostructures. When an AlN intermediate layer is grown on the nitrided surface, under nitrogen rich conditions, AlN grows with a bimodal distribution of nanocolumns. We observe that GaN grown on this AlN intermediate layer prefers to grow only on single crystalline AlN nanocolumns with sizes in the range 80-120 nm. We observe that these nano-features are epitaxial with high crystallinity and demonstrate a thirty times increase in band edge emission compared to GaN film grown on bare sapphire. The threading screw dislocation is seen to propagate through both the AlN layer and GaN nanorods with their morphologies depending on the proximity of the dislocations. Our results show that the shape, structure and mosaicity of GaN nano-features are dependent on the morphology of the AlN intermediate layer.