Causes and mitigation of power degradation in 275-nm-band AlGaN multiple quantum well light-emitting diodes

Al x Ga 1 − x N -based deep-ultraviolet light-emitting diodes (LEDs) operating at 260–280 nm are a sustainable alternative to mercury discharge lamps for sterilization and disinfection of water, air, and surfaces. For accelerating their practical applications, the degradation mechanisms specific to 275-nm band AlxGa1−xN quantum well (QW) LEDs fabricated on a (0001) sapphire substrate were investigated. The optical output power (Po) showed a two-mode degradation: initial degradation by about 20%–30% within the operating time (top) shorter than 100 h and mid- to long-term slow degradation to about 60%–70% for top > 100 h. For elucidating the cause of the slow degradation, layer-selective time-resolved photoluminescence (TRPL) measurements on the QWs and emission microscopy observations were carried out at various top. Because the spectral lineshape of the electroluminescence did not change remarkably and QWs appear undamaged even after the slow degradation, overall degradation is attributed to the decrease in the carrier injection efficiency (ηinjection) to the QWs, most likely due to progressive depassivation of initially H-passivated preexisting nonradiative recombination centers (NRCs), namely, vacancy clusters comprising a cation vacancy and surrounding several nitrogen vacancies, in a Mg-doped Al0.85Ga0.15N electron blocking layer (EBL). During the slow degradation, the areas where the nonradiative recombination in the EBL took place tended to converge in particular zones, indicating a convergence of current paths in low ηinjection zones. By decreasing the Mg doping concentration in the EBL for the purpose of reducing NRC concentration, a device lifetime exceeded 18 000 h, keeping the TRPL lifetime unchanged at least until 16 000 h.