Competing processes in long term accelerated aging of double heterostructure Ga1−xAlxAs light emitting diodes

Abstract

Two competing processes have been observed in bias aging of double heterostructure (DH) Ga1−xAlxAs light emitting diodes (LEDs) at elevated temperatures. These high radiance LEDs are used as sources in optical data links. In the past, only a single aging process, slow degradation in light output, has been reported for dark line defect (DLD)-free LEDs of similar type. In contrast, we report for the first time, a second process which gives a gradual increase in light output. As temperature and current are increased this new process increasingly masks the degradation process, until at 270 °C there is a net increase in light output of 50% after about 300 h of aging. We show that the two processes have different time dependences and activation energies, and that their effects on the light output during aging can be separated mathematically. A model for the new process is presented in which nonradiative traps anneal out in a recombination-enhanced defect reaction. This leads to higher injection efficiency and light output at fixed current. Operating lifetimes increase due to the second process; for example, at 25 °C mean-time-to-failure (MTTF) increases by a factor of three to 9×107 h.