Aluminum gallium nitride(AlxGa1−xN)films withx=0tox=0.36,grown by metal–organic chemical vapor deposition on sapphire, were characterized by cathodoluminescence (CL) imaging and spectroscopy, photoluminescence (PL) spectroscopy, and optical microscopy. In each film, the CL and PL spectra excited from the top surface show a narrow band-edge peak and a broad deep-level peak. In some films, the PL spectrum excited from the film–substrate interface (through the sapphire substrate) shows an additional narrow peak below the band edge, which is attributed to interfacial defects. CL imaging and optical microscopy reveal two types of large-scale defects: networks of lines, which are probably microcracks, and roughly hexagonal raised areas. The hexagonal areas occur only in the lower-xfilms, while the microcrack density increases withx. Spatially resolved CL spectra taken near the large-scale defects show large peak shifts attributed to stress relaxation, and below band-edge peaks attributed to localized states. ©1997 American Institute of Physics.