A GaN-free LED Structure for High UV-light Extraction

Abstract

ABSTRACTWe improved the extraction of ultraviolet light from AlGaN-based light emitting diodes (LEDs) at the wavelength of about 350 nm, by introducing a transparent structure that is free from binary GaN. We used an AlN-template layer on a sapphire substrate as starting medium of the metal organic vapor phase epitaxial growth. The buffer layer is an Al0.2Ga00.8N alloy. We also used a short period alloy superlattice as transparent and conductive p-type cladding and p-type contact layers. The resultant epitaxial structure is confirmed to be transparent with the transmittance of about 90% within the wavelength range of 340 – 400 nm. The crystal quality of the Al0.2Ga00.8N buffer layer was estimated by the transmission electron microscope (TEM) observation. The dislocation density of AlGaN buffer layer was highly reduced on the AlN template layer, both of which have a low density of screw and mixed-type dislocations, of the order of 10−8cm−2. We also found that light extraction is improved by a factor of 6 by introducing the transparent LED structure and a p-contact mirror. The resultant LED shows high performance under RT-CW operation, where 1 mW output power at 348 nm with 20 mA injection corresponds to the external quantum efficiency of 1.4 %. The maximum power was 7 mW at 220 mA. The emission spectrum is highly monochromatic with the UV-to-visible intensity ratio of about 1000. We also demonstrate the application of this transparent UV-LED to white light source in a bottom-emission geometry.