Crystal-field analysis of photoluminescence from orthorhombic Eu centers and energy transfer from host to Eu in GaN co-doped with Mg and Eu

Gallium nitride co-doped with magnesium and europium shows great potential for active layers in red light emitting diode structures due to strong and sharp luminescence emission around 620 nm. In this work, sharp and intense Eu³⁺ luminescence lines from the excited states of the ⁵D_J (J=0, 1) multiplets to the ground states of the ⁷F_J (J=0, 1, 2) multiplets have been analyzed using a C_2v crystal-field equivalent operator Hamiltonian. A model of Eu centers with the C_2v symmetry has been proposed to be an Eu³⁺ complex accompanied by either a pair of nitrogen and gallium vacancies (V_N-V_Ga) or a pair consisting of a nitrogen vacancy and magnesium impurity (V_N-Mg_Ga) in the vicinity of the Eu ion based on the crystal-field analysis, the selection rules and the observed polarization of the Eu³⁺ luminescence lines. Energy transfer from the host to the Eu ions under band-to-band excitation occurs through electron-hole recombination between V_N with the electron-like state and V_Ga or Mg_Gawith the hole-like state; these may be associated with the shallow-trapped or deep-trapped states, respectively, proposed as the energy transfer mechanism in previous literature.