Dynamics of Energy Transfer Mechanism and Excitation Processes in GaN:Eu

A systematic series of combined excitation emission spectroscopy, photoluminescence and time-resolved photoluminescence measurements were performed on the recently developed high efficient red light-emitting diodes to elucidate the concentrations,efficiencies, energy transfer dynamics and excitation processes in Eu-doped GaN. All peaks from photoluminescence are recorded and assigned with related transitions.The concentrations and efficiencies of different defect complexes are determined for selected most efficient samples, the multilayer samples and the multi-quantum well structure. The single thick layer sample is selected as a reference. The single thick layer sample and multilayer samples have more optically active Eu ions than the multi-quantum well structure, while the newly designed multilayers and multiquantum well structure samples show higher efficiencies compared with single thick layer sample.By using time-resolved photoluminescence, it is found that the energy transfer from GaN to the Eu3+ takes place on a timescale of less than 100 ps. For the most efficient Eu defect complexes, this transfer dominantly occurs directly into the 5D0 state of Eu3+. Less efficient complexes also exhibit transfer into the 5D2 state, indicating the importance of the excitation pathway on device efficiency. Under highexcitation intensity, re-excitation can occur, leading to a redistribution of population into the 5D2, 5D1 and 5D0 states. A detailed model is established to explain the above processes. Based on the model, the intensity of output emissions with different wavelengths can be controlled and exploited to achieve the color-tunablility.Time-resolved measurements also reveal that the most efficient Eu defect complexes can be excited and converted to another type of complex by capturing extra electrons. The time to reach equilibrium state of them under different temperatures were determined. Also, the time to break the equilibrium is determined.Moreover, the micro-cavity structure is introduced and the decay from them show accelerations compared with samples without a micro-cavity structure.