We report on a defect related luminescence band at 2.4 eV in aluminum nitride bulk crystals, for which we find strong indications to be related to silicon DX centers. Time resolved photoluminescence spectroscopy using a sub-bandgap excitation reveals two different recombination processes with very long decay times of 13 ms and 153 ms at low temperature. Based on the results of temperature and excitation dependent photoluminescence experiments, the process with the shorter lifetime is assigned to a donor-acceptor pair transition involving the shallow silicon donor state, which can be emptied with a thermal dissociation energy of 65 meV. The slower process with a thermal quenching energy of 15 meV is assigned to the slightly deeper Si DX state known from electron paramagnetic resonance experiments, which is transferred back to the shallow donor state.

We report on a defect‐related broad emission band at around 2.05 eV in bulk aluminum nitride crystals nominally undoped, but containing high concentrations of carbon, oxygen, and silicon. Time‐resolved photoluminescence spectroscopy yields two different exponential decays of this band: A slower process with 1.5 ms lifetime, and a faster process with a characteristic lifetime below 100 ns, which is activated with around 180 meV energy. The slow decay is ascribed to a spin‐forbidden transition between an oxygen‐related DX− S=1 state and a deep acceptor, which for higher temperatures turns into a donor‐acceptor pair transition between the effective‐mass‐like shallow oxygen donor state to the same deep acceptor. The acceptor is tentatively assigned to carbon or a carbon‐related complex.