High-density micro-light-emitting diode (μ-LED) arrays are the key to next-generation ultrahigh-resolution displays. We have demonstrated a full-color LED consisting of Eu-doped GaN and InGaN quantum wells (QWs). The full-color LED exhibited a remarkably wide color gamut and its maximum luminance reached over 3000 cd/m2. However, due to the quantum-confined Stark effects, the wavelength of a green LED shifted to a shorter wavelength as the injected current increased, which resulted in a reduced color gamut. There are two strategies in this regard; the use of a semi-polar substrate with reduced internal electric field and Tb-doped III-nitrides as a green emitter.
Development of an efficient red LED based on GaN is pivotal to ultra-small-size, full-color, and high-resolution micro-LED displays. In a red LED using Eu-doped GaN (GaN:Eu), the peak position of the emission is extremely stable against ambient temperature and injected current. Photoluminescence quantum efficiency of the Eu emission was investigated as a function of chip size of square structures. Even for sizes smaller than 24 µm, an influence of sidewall-related non-radiative recombination of carriers on the quantum efficiency was only minor as a result of limited carrier diffusion lengths in GaN:Eu. We also demonstrated monolithic vertically stacked full-color LEDs consisting of GaN:Eu and InGaN quantum wells. These results indicate a high potential of the GaN:Eu LED for the micro-LED applications.
Eu-doped GaN is a promising material with a wide array of potential applications in optoelectronics, optogenetics, micro displays and quantum computing. While this system has been the subject of intense investigation for the last two decades, several questions still remain about certain aspects of its optical properties, such as the polarization dependence of the optical transitions, and the coupling between the 4f-electron configuration and bulk phonons, as well and the appearance of local phonon modes. Moreover, the origin of certain emission peaks remains under debate in the literature. In this proceeding, the results of a systematic series of “site-selective” photoluminescence measurements are presented, where the properties of pulsed and continuous-wave laser excitation, such as polarization and intensity, were controlled.
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