Defects, performance, and reliability in UVC LEDs

UVC LEDs are of fundamental importance for many applications, including sterilization and disinfection, thanks to their high efficiency and low environmental impact. However, several physical processes still limit the lifetime and reliability of these devices. We present recent case studies in the field of UVC LED reliability. Initially, we review the performance/efficiency of state-of-the-art commercial devices, and discuss the issues related to LED self-heating, and the related electro-optical transient behavior. Then, we discuss the impact of defects on LED degradation, based on combined deep-level transient spectroscopy (DLTS) and deep-level optical spectroscopy (DLOS) measurements, and Technology Computer-Aided Design (TCAD) simulations. We show that, during prolonged operation, UVC LEDs can show considerable changes in the electrical characteristics: a) an increase in the sub-turn on leakage, that can be reproduced by TCAD as due to an increase in trap-assisted tunneling, related to deep traps located in the interlayer between the last barrier and the EBL; b) an increase in the turn-on voltage, that is explained by the degradation of the metal/p-GaN contact, due to a decrease in the active magnesium concentration. Electro-optical measurements reveal that a stronger degradation is detected at low measuring current levels, confirming an important role of defect-mediated recombination. Remarkably, degradation kinetics do not follow an exponential trend, but can be fitted by using the Hill's formula. A higher Mg doping in the EBL mitigates the degradation rate. Results are interpreted by considering that degradation is due to the de-hydrogenation of point defects, which increases the density of non-radiative recombination centers.