Luminescence and energy-transfer mechanisms in Eu3+-doped GaN epitaxial films

We studied the photoluminescence (PL) and energy-transfer processes in Eu3+-doped GaN epitaxial films by means of microscopic PL imaging spectroscopy. Eu3+-doped GaN epitaxial films exhibited blue luminescence due to bound-exciton recombinations in GaN host crystals and red luminescence due to intra-4f transitions of Eu3+ ions. We found an anticorrelation between the exciton and Eu3+ PL intensities in space-resolved PL images, indicating that energy transfer from GaN crystals to Eu3+ ions determines the Eu3+ luminescence intensity. PL and PL excitation spectra showed that efficient Eu3+ luminescence is caused by two different excitation processes: energy transfer from the low-energy charge-transfer (CT) states to Eu3+ ions or from the delocalized states above the band edge of GaN crystals to Eu3+ ions. The energy-transfer process from the CT state to Eu3+ ions dominates the Eu3+ luminescence.