Luminescence of Eu3+ in GaN(Mg, Eu): Transitions from the 5D1 level

Eu-doped GaN(Mg) exemplifies hysteretic photochromic switching between two configurations, Eu0 and Eu1(Mg), of the same photoluminescent defect. Using above bandgap excitation, we studied the temperature dependence of photoluminescence (TDPL) of transitions from the excited 5D1 level of Eu3+ for both configurations of this defect. During sample cooling, 5D1→7F0,1,2 transitions of Eu0 manifest themselves at temperatures below ~200 K, while those of Eu1(Mg) appear only during switching. The observed line positions verify crystal field energies of the 7F0,1,2 levels. TDPL profiles of 5D1→7F1 and 5D0→7FJ transitions of Eu0 show an onset of observable emission from the 5D1 level coincident with the previously observed, but hitherto unexplained, decrease in the intensity of its 5D0→7FJ emission on cooling below 200 K. Hence the 5D0→7FJ TDPL anomaly signals a back-up of 5D1 population due to a reduction in phonon-assisted relaxation between 5D1 and 5D0 levels at lower temperatures. We discuss this surprising result in the light of temperature-dependent transient luminescence measurements of Eu0