Investigation of the light-extraction efficiency in 280 nm AlGaN-based light-emitting diodes having a highly transparent p-AlGaN layer

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The light-extraction efficiency (LEE) of 280 nm AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) with a highly transparent p-AlGaN layer was investigated using a three-dimensional (3D) finite-difference time-domain method. This investigation clearly demonstrated that two major problems need to be solved to obtain an LED structure that guarantees the high LEE for both TE and TM polarized light emissions. The first problem was shown to be light confined in quantum wells (QW) and the n-AlGaN layer. By analyzing a conventional LED structure with a highly transparent p-AlGaN layer and an AlN/sapphire template, the major cause of the light confinement was confirmed as the total internal reflection (TIR) at the interfaces between the EBL, QW, n-AlGaN, and AlN layers. Especially, light confined in the QWs is severe due to TIR at the interfaces adjacent to them. This light diminishes with time due to absorption. By removing the AlN/sapphire layer beneath the n-AlGaN layer, adopting an Al electrode, introducing cone-shaped pillars on the n-AlGaN layer and having cylindrical holes through the Al electrode, LEEs of 54.3% and 49.3% were attained for the TE and TM polarized emissions, respectively. This result demonstrates that just by offering proper light escape angles directly to the light confining layers, attainment of LEEs of more than 50% is possible. The second problem is severe light absorption by the metal electrode. LEE vs. the reflectivity of the metal electrode was analyzed for several LED structures. In the extracted results, an exponential increase in LEE as the metal reflectivity increases was shown, which means reflection at the interface between the p-AlGaN layer and the metal electrode is repeated several times until the light escapes from the LED. Also, it was shown that once the reflectivity of the metal electrode is more than 90%, any additional small increase in reflectivity increases LEE significantly. These results emphasize that without high reflectivity of the metal electrode and without releasing the light confined in several of the epitaxial layers, attaining LEEs of over 70% is difficult for AlGaN-based LEDs operating at 280 nm.