Driven by a wide range of applications, as shown in Fig. 1(a) , the development of AlGaN-based light emitting diodes in the deep ultraviolet spectral range (DUV-LEDs) has greatly intensified. In contrast to conventional ultraviolet sources UV-LEDs exhibit small form factors, operate at moderate dc voltages, show long lifetimes, and their emission covers the entire UVB and large parts of the UVC wavelength range. Currently, the main driving force are high volume applications like water purification, disinfection of surfaces and appliances, inactivation of germs in air treatment systems as well as sterilization of medical equipment with a focus on UVC-LEDs emitting near the germicidal effectiveness peak around 270 nm [1] . This presentation will provide an overview of state-of-the art in DUV-LED device technologies and present recent advances in the development of high quality AlGaN materials by metalorganic vapour phase epitaxy (MOVPE). We will discuss different approaches to improve the internal quantum efficiency of UV light emitters, including the growth of low defect density AlN layers on sapphire substrates by epitaxial lateral overgrowth (ELO) as well as sputtered and high temperature annealed AlN on sapphire [2] . We will demonstrate AlGaN quantum well (QW) LEDs with output powers of more than 50 mW for single-chip emitters near 265 nm and explore the wavelength limits of deep UV-LEDs with emission as short as 217 nm. These far UV-LEDs are ideally suited for sensing applications like the monitoring of combustion engines, toxic gases, nitrates in water, and may also be utilized for the in-vivo inactivation of multi-drug-resistant germs or viruses without damaging the human skin. Although a strong decline can be observed in the external quantum efficiency (EQE) of UV-LEDs emitting below 250 nm, as shown in Fig 1(b) , considerable advances have been made in the development of 233 nm LEDs with output powers of nearly 1.9 mW at 100 mA corresponding to an EQE of 0.35% [4] . We will discuss the root causes for the EQE decline, including changes in the optical polarization of light emission from deep UV AlGaN QWs and their effects on light extraction as well as changes in internal quantum efficiency and carrier confinement. We will provide an outlook of future progress in DUV-LED performance and demonstrate first applications including a spectrally pure 233 nm irradiation system for the in-vivo inactivation of germs.
