We have investigated by transmission electron microscopy and photoluminescence spectroscopy the impact of the structural features on the optical properties of (Ga,In)(N,As) quantum wells emitting in the 1.3 to 1.55 Am wavelength range. Our electron microscopy analysis demonstrates morphological instabilities in the (Ga,In)(N,As) layers with high In and N concentrations. We show that the optical properties are strongly influenced by the actual microstructure (interface roughness, composition variations). High photoluminescence efficiency is only achieved for quantum wells grown in the two-dimensional mode. Ex situ annealing of those perfect quantum well structures can still improve their luminescence, and the concomitant structural changes are detected as local rearrangements of N and In atoms inside the well. (C) 2004 American Vacuum Society.