Growth of GaN Hexagonal Arrays on Partially Crystallized Sapphire Nanomembranes for Micro-Light-Emitting Diodes

100-nm-thick hexagonal fully and partially crystallized sapphire nanomembrane arrays were demonstrated as growth templates for hexagonal micro-sized GaN arrays for micro-light-emitting diodes (LEDs). By simply controlling the thermal treatment time for crystallization of the amorphous membrane, the partially crystallized nanomembrane which has both single- and polycrystalline alumina on the top surface was fabricated. The unique crystalline structures contributed to unique growth behavior which contains selective and lateral overgrowth of GaN, compared to that of GaN on fully crystallized sapphire nanomembranes, leading to superior structural and optical properties of GaN. Threading dislocation densities were reduced by 75% and 1.29 times higher integrated photoluminescence intensity was achieved compared to GaN on the fully crystallized sapphire nanomembranes. Stress in GaN was also fully relaxed due to the ultrathin membrane structure as a compliant substrate. The superior properties of GaN as well as the truncated inverted pyramid structure are expected to improve the performance of micro-LED devices. Hexagonal partially crystallized sapphire nanomembrane technology also provided advances in the fundamental study of epitaxial growth of GaN-based materials.