Rare-earth (RE) doped III-nitrides, prepared by in-situ doping during growth
or by ion implantation and annealing, are promising materials for visible
light emitting displays. In addition, they are extremely challenging
theoretically, on account of the complexity of the sharp inter-4f optical
transitions, which are allowed only through the mixing by
non-centrosymmetric crystal fields of the inner 4f orbitals with
higher-lying states of opposite parity. We review recent experimental and
theoretical work on Er-, Eu- and Tm-doped III-nitride compounds and alloys
which has been carried out with a view to establishing the lattice location
of RE in these materials and the probable nanostructure of the centres which
are responsible for their luminescence. The isolated site REIII is
found to be both optically and electrically inactive, but in association
with neighbouring intrinsic defects (most probably nitrogen vacancies)
REIII can generate a small family of similar optically active sites.
Such a family is held to be responsible for the site multiplicity that is a
common feature of the spectroscopy of RE-doped III-nitrides.