Re-Excitation of Trivalent Europium Ions Doped into Gallium Nitride Revealed through Photoluminescence under Pulsed Laser Excitation

The behavior of trivalent europium (Eu³⁺) ions doped into gallium nitride (GaN) was investigated under intense excitation conditions to explore the excitation energy transfer characteristics in the presence of large carrier densities. Under such conditions, strong emission from the higher excited ⁵D₁ and ⁵D₂ states of the Eu³⁺ ions was observed in highly efficient AlGaN/Eu-doped GaN multiple quantum wells grown by organometallic vapor phase epitaxy. This behavior was studied using a variety of excitation sources and conditions. Most notably, when a femtosecond-pulse laser was used, the excitation of the Eu³⁺ ions into the higher energy states became significant only with a second excitation pulse arriving within the lifetime of the ⁵D₀ state. We propose that an already excited Eu³⁺ ion is promoted from its ⁵D₀ excited state into the higher ⁵D_J states where it relaxes and can emit from the ⁵D₁ and ⁵D₂ states.