  We study the multi-exciton optical spectrum of vertically coupled GaN/AlN
quantum dots with a realistic three-dimensional direct-diagonalization approach
for the description of few-particle Coulomb-correlated states. We present a
detailed analysis of the fundamental properties of few-particle/exciton
interactions peculiar of nitride materials. The giant intrinsic electric fields
and the high electron/hole effective masses give rise to different effects
compared to GaAs-based quantum dots: intrinsic exciton-exciton coupling,
non-molecular character of coupled dot exciton wavefunction, strong dependence
of the oscillator strength on the dot height, large ground state energy shift
for dots separated by different barriers. Some of these effects make GaN/AlN
quantum dots interesting candidates in quantum information processing.
