Comparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor deposition

Abstract

Microstructures of low-temperature GaN and AlN nucleation layers (NLs) grown on sapphire by metalorganic chemical vapor deposition and changes in these microstructures and surface morphologies upon annealing at high temperature (HT) were investigated by transmission electron microscopy and atomic force microscopy. Upon annealing, both NLs undergo changes using different pathways, resulting in islands submerged in continuous GaN and AlN NLs. Possible explanations for the rearrangement of the microstructure during annealing of the NLs are suggested for each case. The step flow growth of HT-GaN on GaN NLs initiates directly from the step ledges of the existing rounded nucleation islands, while nucleation of GaN islands on the undulated surface precedes the step flow growth on AlN NLs. The earlier appearance of flat-top islands on AlN NLs was accounted for by localized growth enhancement due to the surface energy difference between AlN and GaN.