Degradation mechanisms beyond device self-heating in deep ultraviolet light emitting diodes

Lifetime measurements on single, packaged UV LEDs were performed under constant current injection at 20 and 75 mA (60 and 226 A/cm²). The junction temperature at operation was found by micro-Raman spectroscopy to be 57 and 184degC, respectively. Unbiased LEDs of similar characteristics placed in an oven baked at the equivalent operating junction temperatures showed a degradation in output power similar to that in the current injection devices during the initial 24 hours, but did not continue to degrade beyond that time. These studies imply that device heating, is correlated with the initial drop in output power during burn-in, but is not directly linked to the total degradation over the lifetime of the device. Time-resolved PL studies on the device active region as well as further electro-optic measurements indicate that the degradation is not due primarily to that of the active region, but may be associated with generation of point defects such as N-vacancies near the p-n junction.