  A theoretical interpretation of the photoluminescence excitation spectra of
self-organized polar GaN/(Al,Ga)N quantum dots is proposed. A numerical method
assuming a realistic shape of the dots and including the built-in electric
field effects is developed to calculate their energy structure and hence their
optical absorption. The electron and hole spectra show the existence of a set
of quasi-bound states that does not originate from the wetting layer, and plays
a crucial role in the observed absorption spectrum of the GaN/(Al,Ga)N dots.
Transitions involving these quasi-bound states and wetting layer states give a
sufficient explanation for the observed continuum absorption background. The
properties of this absorption band, especially its extension, depend strongly
on the dot's size. Our simulation provides a natural explanation of the
experimental luminescence excitation spectra of ensembles of dots of different
heights. Our theoretical model can be extended to cases where the confinement
potentials are complicated by the presence of a stronger electric field.
