Defects and reliability of UVC-LEDs

Despite their widespread industrialization, the wallplug efficiency (WPE) and reliability of AlGaN-based UVC-LEDs remains below par in comparison to blue light emitting diodes. While the WPE and external quantum efficiency (EQE) can be significantly improved by enhanced photon extraction schemes, the lifetime of these relatively new solid-state light sources is limited by several optical and electrical degradation processes. For LEDs emitting at 265 nm, previous literature reports have demonstrated that during the early stages of operation, hydrogen instability plays a fundamental role, since it is prone to dissociate from already-present vacancy complexes that once ionized act as non-radiative recombination centers (NRRCs), thus lowering the optical efficiency of the device. At lower injection levels, this decrease was also found to well correlate with the increase in trap-assisted tunneling (TAT) conduction at sub-turn-on forward bias. This indicates that the process responsible for the generation of deep-levels between the EBL and the active region, is also causing the decrease in optical efficiency. Numerical simulations confirm this hypothesis, also showing that the detected degradation trends are compatible with the relocation of hydrogen originating from the p-side of the device. These degradation mechanisms are also observed in 233 nm LEDs, which are gaining attraction because of their combined germicidal effectiveness and minimal risk to human skin. Specific details on the ongoing degradation processes are investigated with the help of color-coded LEDs by means of deep-level optical spectroscopy and numerical simulations.