Surface step morphologies of GaN films grown on vicinal sapphire (0 0 0 1) substrates by rf-MBE

Surface step morphologies of GaN films grown on vicinal sapphire (0 0 0 1) substrates are studied in molecular beam epitaxy. Using atomic force microscopy, monolayer and multi-layer steps are clearly observed. It is found that step morphologies greatly depend on the vicinal angle and the inclination directions of the substrate. Step bunching on the surface during the growth occurs when the vicinal angle is larger than 1.0∘1.0∘. Furthermore, straight and zigzag steps are formed on the surface with the inclination direction of the vicinal surface toward a-axis and m-axis of the sapphire (0 0 0 1) substrates, respectively. An atomic-scale model is considered to explain the anisotropic step morphologies.