We have used low energy electron-excited nanoluminescence (LEEN) spectroscopy to probe the localized electronic states at GaN free surfaces, metal-GaN contacts, and GaN/InGaN quantum well interfaces. These depth-resolved measurements reveal the presence of deep electronic states near GaN interfaces whose energies and relative densities depend sensitively on the local chemical structure and growth conditions. The physical properties of these states correlate with mobility variations in thin GaN films grown by molecular beam epitaxy, Fermi level positions at Mg and Al/GaN Schottky barriers, and the appearance of new phases localized near GaN/InGaN/GaN quantum well interfaces. The growth and processing dependence of deep GaN levels highlights new methods to understand and control the fundamental electronic structure of GaN heterointerfaces. © 2000 Elsevier Science S.A. All rights reserved.