Formation mechanism and suppression of Ga-rich streaks at macro-step edges in the growth of AlGaN on an AlN/sapphire-template

•Formation mechanism of Ga-rich streaks at macro-step edges of AlGaN on AlN/sapphire.•Macro-steps formed during AlN growth are maintained through upper AlGaN.•Less inter-surface diffusion between the facets of macro-step edges and c-plane.•Ga-rich streak formation can be explained by higher Ga incorporation efficiency at facets.•AlGaN growth without Ga-rich streaks was achieved by flattened AlN-template. The formation mechanism of Ga-rich streaks in the AlGaN layers formed at macro-step edges was investigated by analyzing AlGaN layers grown by metalorganic vapor phase epitaxy on AlN/sapphire-templates. Energy dispersive X-ray spectroscopy in the cross-sectional scanning transmission electron microscopy revealed that the macro-steps formed during AlN growth were maintained through the upper AlGaN layer. This can be explained by the less interaction between adjacent macro-steps due to the sufficiently smaller surface diffusion lengths of Ga and Al species than the inter-macro-step distance of submicron to a few microns. Moreover, atomic force microscopy indicated that the apparent smooth region between macro-steps involved many atomic steps and terrace width of a few ten nanometers which may be the actual scale of the diffusion lengths. At the macro-step edge, facets that are different from a c-plane are formed. The higher incorporation efficiency of Ga atoms on these facets compared with that of the c-plane likely led to the formation of Ga-rich streaks due to the less inter-surface diffusion between the facets and c-plane regions. We demonstrated AlGaN growth without Ga-rich streaks using the flattened AlN/sapphire-template by polishing; this resulted in the achievement of homogeneous emission energies in AlGaN-based quantum wells.