Fundamental aspects of Deep UV light emitting diodes and failure reduction of LEDS grown on AlN Substrates
Rajul Randive, and Leo J. Schowalter , Crystal IS
Abstract: Light-emitting diodes (LEDs) in the mid-ultraviolet wavelength range of 250 to 285 nm have demonstrated reliability and lifetime suitable to enter the commercial markets of instrumentation, water purification, and biodetection. we will discuss the basics of UVC LEDs and their benefits over other methods of disinfection; the basics of UV disinfection (including log reduction and dosage requirements); and common target microbes for these applications. Through the pseudomorphic growth of AlGaN heterostructures on AlN substrates, low-defect density LEDs are now available with improved efficiencies and increased reliability. An investigation of acceleration parameters is needed for further improvement in device reliability. In this talk, two different LED package designs, the higher thermally resistant Optan TO-39 package and a lower resistant Optan SMD package, are subjected to accelerated lifetime testing. Common acceleration factors of temperature and current are investigated, with currents ranging up to 400 mA and case temperatures from -40 deg C to 85 deg C. The rate that output power degrades with time is found to depend primarily on the semiconductor junction temperature. In particular, junction temperatures higher than approximately 115 deg C are observed to cause increased L50 failures within 1,000 hours of operation. However, leakage currents at low or reverse voltages, a common cause of device failure, are dependent on operating current. Infrared emission microscopy was used to identify the areas of increased leakage current, from which transmission electron microscopy of the locations was performed to gain insight into the cause of this failure mechanism. This analysis showed that increased leakage current was originated in areas that, when not completely destroyed due to an extremely localized high current density, were centered on dislocations beginning in the AlN substrate. These dislocations generate locally high dislocation densities in the epitaxial layers, affecting the device structure and providing other paths for carrier recombination.