Color tunability of light-emitting diodes based on Eu-doped GaN active layers

In the pursuit of color-tunable single-pixel LEDs, it has been demonstrated that under current injection, a Europium-doped Gallium Nitride (GaN:Eu) device can be tuned from red to orange to yellow by adjusting the duty cycle and amplitude of pulse-width modulated pulsed excitation. The underlying process is a redistribution of excitation by the re-excitation of already excited centers. To further study the underlying processes, modeling has been performed by considering a local defect complex consisting of a trap for carriers and a Eu ion. The transfer of energy within the defect complex is modeled with a rate equation approach using experimentally determined constants such as transfer rates and lifetimes. Different pulse sequences and carrier densities per unit time are used to simulate experimental conditions. The model predicts the relative average populations of the Eu’s ⁵D₀ and ⁵D₁ states. These populations can be related to chromaticity using the color mixing of the red emission from the ⁵D₀ state with green emission originating from the ⁵D₁ state in order to predict the generated color. We present a comparison of the predicted and experimentally observed behaviors and discuss extensions of the model and possibilities to extend the color coverages of the single-pixel LED.