Optical signatures of silicon and oxygen related DX centers in AlN

Abstract

Bulk AlN crystals typically contain high concentrations of oxygen, silicon, and carbon − 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 = 1) DX-centre state of the donors, a shallow EMT-like conventional donor state, or a free electron − 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.