Enhanced photon extraction efficiency in 260nm pseudomorphic AlN-based ultraviolet light emitting diodes

Light sources in the wavelength range below 300nm have attracted extensive attention due to their applications in instrumentation and in disinfection of water, air and surface. Nitride-semiconductor-based, light emitting diodes (LEDs) are of especially great interest due to numerous advantages compared to conventional mercury lamps. Although significant progress of more than 9mW of quasi-CW power around 260nm at room temperature has been achieved from devices with Al_xGa_1-xN heterostructures pseudomorphically grown on native AlN substrate [1], those devices still have relatively low photon extraction efficiencies (estimated to be around 4%). In particular, half of the generated photons are directed towards the p-contact and absorbed by the smaller bandgap p-GaN. The other half of the photons, which are directed toward the exit surface, will experience absorption in AlN substrate due to point defects which create states within the bandgap of the AlN material. In addition, only a small fraction of those photons actually reaching the exit surface will escape due to the relatively large refractive index difference between AlN and air resulting in a narrow escape cone.