AlGaN metasurface to increase the light-extraction efficiency of deep ultraviolet light-emitting diodes by perfect transmittance before critical angle

In this study, we investigate AlGaN metasurface increasing the light-extraction efficiency (LEE) of AlGaN-based deepultraviolet light-emitting diodes (DUV-LEDs) by utilizing the finite-difference time-domain (FDTD) method. As a first step, a unit cell of metasurface structure adopting the AlGaN cylindrical resonator, which highly increases the transmittance near 280 nm wavelength before the critical angle, was searched numerically. A selected unit cell structure with the resonator was lattice constant a =110 nm with square lattice, the height of cylinder h = 45 nm, and radius r = 40 nm. Transmittance map was constructed for the optimized unit cell as functions of wavelength and incidence angle of plane waves for TE and TM polarized sources, respectively. The map showed perfect transmittance near 280 nm wavelength with normal incidence. Angle-dependent transmittance slowly decreases as the incidence angle increases, but as the incidence angle positions near the critical angle, the decrease of transmittance is gradually accelerated. As a next step, the extracted AlGaN metasurface structure is uniformly deployed to a flip chip LED, and light-extraction efficiencies are calculated as a function of p-GaN thickness for TE and TM mode sources, respectively. The dimension of LED considered in this study was about 2μm× 2μm×1μm. Calculated LEE values clearly showed that by adopting the designed AlGaN metasurface, LEE always increases regardless of p-GaN thickness based on the fundamental increment of transmittance.