Inverse Tapered AlGaN Micropillar and Nanowire LEDs for Improved Light Extraction Efficiency at 270 nm

Deep-ultraviolet (DUV) light emitting diodes (LEDs) show great potential in a wide range of applications, but suffer from poor external quantum efficiencies (EQEs) when compared to visible LEDs, due to their exceptionally poor light extraction efficiency (ηEXT). One successful method of improving the ηEXT of DUV LEDs is the use of micropillar and nanowire arrays. Our previous works have reported the development of an “inverse taper” profile in AlGaN micropillars during wet etching in heated hydroxyl-based chemistries. Here, we study the effects of inverse tapering on the ηEXT of AlGaN microstructures and nanostructures at 270 nm using finite-difference time-domain simulations, in accordance with our experimental results. Results show that ηEXT can be increased from ∼45% to more than 95% for TM-polarized emission in microstructures with diameters of 1 μm and heights >1 μm, and from ∼35% to more than 85% for TE-polarized emission by an inverse taper angle of 5 °. Results for nanostructures also indicate significant ηEXT improvements through tuning of the inverse taper angle. These findings, along with our demonstration of inverse tapered high Al-content AlGaN micro and nanostructures, could enable the development of record-high EQE DUV LEDs based on arrays of high aspect ratio structures with high ηEXT.