Efficiency Limits in Coalesced AlGaN Nanowire Ultraviolet LEDs

Nanowire AlGaN III-nitride LEDs are claimed as potential high-efficiency solid-state photon sources spanning to the short-wavelength deep ultraviolet (UV). Nanowire LEDs (NWLEDs) emitting in the UV are compared with a transparent n-AlGaN top electrode formed by coalescing the top region of nanowire-ensemble LEDs with commonly employed opaque conformal metallic electrodes used for nanowire-based devices. The use of a transparent contact results in an increase in the wall plug efficiency of >25x, exceeding the expected increase due to enhanced photon-extraction efficiency. Increased nanowire connectivity reduces the short-circuit pathways, enabling higher device yields of relatively large-area (>1 mm(2)) UV nanowire-ensemble LEDs. Despite these large relative improvements, the absolute output efficiency remains miniscule (<1 m%). Electroluminescence microscopy demonstrates that <0.1% of nanowires within the ensemble contribute to emission. The single-nanowire efficiency is estimated and points toward improvement of the homogeneity of the injection current as a crucial step for realizing commercially viable UV NWLEDs.