Charged excitons and excitons bound to neutral impurities in wurtzite semiconductor structures

We have established the possible symmetries for the states of charged excitons (trions) in bulk materials with the wurtzite structure and in wurtzite-based heterostructures. We considered both free trions and bound ones (or excitons bound to neutral, i.e., un-ionized impurities). From the Pauli principle for identical fermions it is shown that only one symmetry is possible for the ground state of positively charged free trions involving two holes from the same valence band. The property also holds for negatively charged free trions, except in quantum wells and some superlattices for trions whose hole lies in the A valence band. We established the selection rules for trion optical transitions. Only a few trions are dark. Even more, for many trions, several channels are available, in terms of symmetry, for radiative decay, which suggests fast recombination processes. A comparison is provided between exciton and trion transitions. The changes of the symmetry of the states and of the optical selection rules induced by an electric field parallel to the c axis have been considered. When one (several) phonon(s) is (are) involved in a process, it is always possible to connect any initial state to any final one by an optical transition.