Low-energy electron-excited nanoscale-luminescence (LEEN) spectroscopy of GaN/InGaN/GaN double-heterojunction structures reveal the formation of electronic states localized near the quantum well interfaces under relatively In-rich conditions. These states are due to formation in a cubic GaN region comparable to the quantum well layer in thickness rather than the bulk native defects typically associated with growth quality. The nanoscale depth dependence of the noncontact, nondestructive LEEN technique enables detection of this competitive recombination channel within a few nanometers of the "buried" heterojunction interfaces. © 1999 American Institute of Physics.