In this study the optical properties of high quality c-plane AlGaN layers grown on c-plane sapphire by metal organic vapor phase epitaxy have been investigated. Submonolayers of SiNx have been deposited in situ to reduce the dislocation density. After subsequent AlGaN growth atomic force microscopy shows hexagonal hillocks. They consist of differently oriented facets, which contain different amounts of Al as we find in low temperature scanning electron microscope cathodoluminescence measurements. In macroscopic photoluminescence measurements, this leads to doublet emission bands. Further AlGaN overgrowth planarizes the surface, both emission bands coalesce, and the defect density is reduced.