GaN-free transparent ultraviolet light-emitting diodes

By introducing a GaN-free layer structure, we have transparent light-emitting diodes (LEDs) at the ultraviolet emission wavelength of 348–351 nm, which is shorter than the GaN band gap wavelength of 363 nm. The buffer layer consists of an AlGaN alloy directly grown on an AlN template layer on a sapphire substrate, and a short period alloy superlattice is adopted as p-type cladding and p-type contact layers. The transparency of the epitaxially grown layer structure is confirmed from transmission spectra. The output powers of the device are 1 mW at injection currents of 20 and 7 mW at 220 mA under room temperature continuous wave operation. The highest external quantum efficiency is 1.4%. This value is superior to that of an ultraviolet LED grown on a high-quality bulk-GaN substrate, where the performance was significantly deteriorated by light absorption into the GaN substrate. The results here indicate the importance of a transparent device structure free of GaN to improve the performance of ultraviolet L...