We report the results of room-temperature cathodoluminescence (CL) and of scanning CL and electron (SEM) microscopy of GaN/InGaN structure with a single InGaN quantum well on top. The structures were grown by MOCVD on sapphire with low-temperature (LT) GaN buffer. Depth-profiling CL investigations were used to identify the observed CL emissions, which show a complicated in-depth evolution. The influence of a LT GaN buffer on structural and optical properties of GaN/sapphire interface in the structure is discussed. Our results show that inter-diffusion of Al from sapphire to the GaN buffer layer takes place. A gradual improvement of film quality with increasing distance from interface is demonstrated.