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.

Bulk AlN crystals typically contain high concentrations of oxygen, silicon, and carbon À as also state-of-the art epitaxial layers typically do, depending on the specific growth conditions. In optical spectroscopy, such crystals show broad bands in the region from 2-5 eV in absorption and emission. We investigated several emission bands in the range from 1.4 to 1.9 eV, especially those centred at 2.0 and 2.4 eV, which under below-bandgap excitation with a 325 nm laser dominate the photoluminescence (PL) spectra both at low and at room temperature. We find clear indications that all these transitions occur between different states of the shallow donors Si or O, and deep acceptors. The donors undergo lattice relaxation and form DX centres. Depending on temperature, the initial state is either a long-lived (S ¼ 1) DXcentre state of the donors, a shallow EMT-like conventional donor state, or a free electron À with markedly different relaxation times for the optical transitions under below-bandgap excitation. The acceptor involved in these PL bands is likely linked to aluminum vacancies. Based on our data, we develop configuration coordinate diagrams and a combined level scheme for multiple transitions in the range from 1.4 to 2.4 eV.Tentative level scheme for PL bands observed in AlN in the range from 1.4 to 2.4 eV.

We report on defect‐related photoluminescence bands in the range from 1.4 to 2.4 eV in aluminum nitride bulk crystals and layers. Using continuous photoluminescence, photoluminescence excitation, and time‐resolved photoluminescence spectroscopy, we assign these bands to donor–acceptor pair transitions between shallow donor states or related slightly deeper DX− states of silicon or oxygen donors, and three different types of deep acceptors. These three different acceptors are most likely a (VAl−2false(ON)) complex, a (VAl−ON) complex, and an aluminum vacancy VAl, or different charge states of these.

No abstract