Enhanced conversion efficiency and tailored radiation patterns assisted by photonic-crystal light-extractors in compact MQW based color-converters for µLED applications

State-of-the-art RGB microdisplays use quantum-dot color converters (CC), which suffer from photostability issues and low blue-light absorption. Inorganic MQW based CC offer higher blue-light absorption and better photostability. However, their practical use for microdisplays has not been implemented yet because of their low light extraction efficiency (LEE) inherent to their high refractive indices. In this paper, we investigate the use of photonic crystals (PhC) with different optogeometrical parameters to fully optimize AlGaInP/InGaP MQW CC for blue-to-red and green-to-red color conversions in microdisplays. A 600nm-thick CC was successfully bonded on a transparent substrate using oxide-mediated molecular bonding. By using optimized photonic-crystal designs, we obtained a large LEE enhancement (x 9) within ultra-short extraction lengths (~2μm), which shows quasi-perfect light outcoupling and compatibility with pixel lateral sizes under 5μm. Experimental results are in agreement with 3D-FDTD simulations, showing that those unique characteristics are paired with highly directional emission. A phenomenological model derived from the standard coupled-mode theory has been proposed and used to determine the mean coupling strengths between the guided Bloch modes and radiated modes. We believe that the design guidelines set in this work could pave the way for the use of inorganic MQW CC to achieve monolithic integration for full-color microdisplay applications.