While InGaN/GaN blue and green light-emitting diodes (LEDs) are commercially available, the search for an efficient red LED based on GaN is ongoing. This is essential for the monolithic integration of the three primary colors and the development of nitride-based small-size, full-color, high-resolution displays. We have succeeded in the realization of a red LED using Eu-doped GaN (GaN:Eu) grown by organometallic vapor phase epitaxy [1]. Making use of the intra-4f shell transitions of Eu³⁺ ions, our LED features an ultra-narrow linewidth of less than 1 nm at room temperature, and unprecedented thermal stability of 1 pm/K. By gradual optimization of the Eu excitation mechanism, we have increased the light output power above 1 mW at 20 mA [2].

One of limiting factors for enhanced light output power is the relatively long radiative lifetime of the Eu emission in GaN:Eu (∼300 µs). According to the Fermi’s golden rule, modifying the spontaneous emission rate of Eu ions can be achieved by increasing the photonic density of states (PDOS) at the frequency of spontaneous emission, as already demonstrated with a planar Fabry-Perot cavity. We have boosted the output power by actively manipulating radiative recombination probability at the atomic level of the Eu ions, which can be achieved through control of their photon fields in micro- and nano-cavities. In a GaN:Eu layer embedded in a microcavity consisting of an AlGaN/GaN distributed Bragg reflector (DBR) and a Ag reflecting mirror, a 21-fold increase of the Eu emission intensity was obtained under optical pumping at room temperature [3]. Furthermore, in a preliminary LED with a microcavity consisting of ZrO₂/SiO₂ and AlInN/GaN DBRs, the output power was enhanced by 5 times [4]. In this talk, current status of the red LED and strategies for further enhancement of the light output power are reviewed.

1. A. Nishikawa, T. Kawasaki, N. Furukawa, Y. Terai, Y. Fujiwara, Appl. Phys. Exp. 2, 071004 (2009).
2. B. Mitchell, V. Dierolf, T. Gregorkiewicz, and Y. Fujiwara, submitted to J. Appl. Phys.
3. T. Inaba, D. Lee, R. Wakamatsu, T. Kojima, B. Mitchell, A. Capretti, T. Gregorkiewicz, A. Koizumi, Y. Fujiwara, AIP Adv. 6, 045105 (2016).
4. T. Inaba, K. Shiomi, J. Tatebayashi, and Y. Fujiwara, in preparation.