Unique properties of photoluminescence excitation spectra in a Eu-doped GaN epitaxial film

We have investigated the temperature dependence of photoluminescence-excitation (PLE) spectra of Eu3+ emission due to the intra-4f shell transitions in a Eu-doped GaN epitaxial film from the viewpoint of the energy transfer process by carriers and excitons from the host GaN to Eu3+ ions. It was found that the excitonic band of the PLE spectrum disappears in a low temperature region below ∼140 K in spite of the fact that the optical transitions of the A and B excitons are clearly observed in a reflectance spectrum. The excitonic PLE band becomes remarkable with an increase in temperature. This fact indicates that carriers originating from the thermal dissociation of photogenerated excitons contribute to the Eu3+ emission. In other words, excitons play no role in the energy transfer process. Furthermore, the PLE spectrum at room temperature exhibits an oscillatory structure resulting from longitudinal-optical phonon emission in a hot carrier relaxation process.