GaN/AlN quantum-dot superlattices grown by molecular-beam epitaxy on silicon (111) or sapphire (0001) substrate have been investigated using high-resolution transmission electron microscopy, photoluminescence, and photo-induced absorption spectroscopy. Under interband excitation at lambdaapproximate to351 nm, three resonances are observed, respectively peaked at 2.1 mum (2.36 mum), 1.46 mum (1.69 mum), and 1.28 mum (1.27 mum) for the sample grown on silicon (sapphire) substrate. We show that the absorptions involve conduction-band interlevel transitions from the ground state to p-like or d-like states, and that their energy is governed mainly by the magnitude of the internal field in the GaN dots. (C) 2003 American Institute of Physics.