Identifying the source of deep defect luminescence bands in AlN and GaN: Slowly decaying DX center related emissions

The direct semiconductors AlN and GaN are the materials of choice for optoelectronic applications in the VIS to UV range, but are still hampered by high densities of crystalline defects and a lack of control on unintentional dopants. Especially for AlN, typically broad photoluminescence emission bands are observed over the whole photon energy range from 1.4 – 4.5 eV.
We investigate several of these bands in the range from 1.4 – 2.4 eV being the dominant emission under below-bandgap HeCd laser excitation for both bulk crystals and epitaxial layers. Using different spectroscopic methods like cw and time resolved PL at varying sample temperature, transmission, PL-excitation spectroscopy etc., and find clear indications of the involvement of (relatively shallow) DX states of oxygen and silicon. The capture of free electrons into these states and their thermalization properties are analyzed, and their relation to literature data on Hall and optically detected EPR is discussed. The deep acceptors acting as final states in the donor-acceptor pair-type PL transitions are most likely related to Al vacancies and their complexes with oxygen.
For GaN bulk crystals doped with Beryllium and also unintentionally co-doped with oxygen, a broad PL band coinciding with the yellow PL at around 2.15 eV is found. This PL band has even at room temperature an exceptionally long lifetime of ≈ 1 sec, which by thermal activation with an apparent energy of ≈ 0.26 eV becomes much shorter for temperatures above 300 K. Gated PL spectra again indicate a donor-acceptor pair transition, with both DX-like and EMT-like donor states involved, and a deep acceptor – presumably carbon.