The origins of the DX center formation in unintentionally doped GaN epilayers grown on sapphire substrates

Photoconductivity (PC) and photoluminescence (PL) measurements on unintentionally doped GaN epilayers grown on sapphire substrates by using plasma-assisted molecular beam epitaxy were performed to investigate the origins of the DX center in the GaN layers. Temperature-dependent PL measurements showed a shallow-deep transition behavior. The capture barrier height of the donor by the electron was 64 meV, and the transition energy between the donor and the deep center and the potential barrier height between the shallow level and the deep level were −0.51 and 0.574 eV, respectively. These results indicate that the DX center in unintentionally doped GaN epilayers originate from the oxygen impurity.