Researchers at the The University of Osaka, in collaboration with Ritsumeikan University have shown how growing europium-doped gallium nitride (Eu-doped GaN) on a semipolar crystal plane dramatically improves red light emission.
The researchers say that EU-doped GaN is a promising next-generation microLED material platform, as it provides narrow-linewidth, wavelength-stable red emission based on intra-4f-shell transitions of Eu ions. In this research, it was found that growing these on a semipolar crystal plane selectively promotes the formation of highly efficient Eu luminescent centers, resulting in red emission intensity more than 3.6 times higher than that of a conventionally grown material.
The researchers explain that conventional growth on polar (0001) GaN has a major drawback: many low-efficiency Eu luminescent centers form unintentionally, limiting light output. Using combined excitation-emission spectroscopy, the team showed that low-efficiency centers associated with Eu clustering, OMVPE1 and OMVPE2, were absent in semipolar GaN:Eu. At the same time, the highly efficient center OMVPE7 and another center, OMVPE8, increased dramatically, by factors of 139 and 53, respectively. The resulting semipolar sample exhibited a narrower emission linewidth than the conventional sample, indicating that the brighter emission arose from changes in luminescent-center populations rather than from improved light extraction.
The researchers further suggest that enhanced oxygen incorporation during semipolar growth plays a central role in this effect. Oxygen incorporation was higher in the semipolar sample than in the conventional sample, and this is thought to suppress Eu clustering while favoring local structures related to the highly efficient OMVPE7 center.
Importantly, the advantages were not limited to weak excitation conditions. The semipolar GaN:Eu sample also showed suppressed efficiency droop under strong excitation, meaning that the emission remained comparatively robust as excitation power increased. Overall, the semipolar material delivered a 3.6-fold enhancement in emission at the maximum excitation power density used in the study.
