Reliability of GaN-based LEDs

We report the results of a reliability evaluation plan carried out on GaN/InGaN blue LEDs, including life tests at RT at three current levels: 20 mA, 50 mA, 100 mA, thermal storage at high ambient temperature (up to 300°C), and finally high current density (≈100 A/cm2 ) and high junction temperature (300 °C). Failure modes consist of: (a) an increase in series resistance and/or forward voltage; (b) an increase in the tunneling component of forward diode current, and of reverse current; (c) a substantial degradation of optical intensity. Device degradation is correlated with a remarkable decrease in the apparent doping and with the appearance of deep levels having activation energies of 0.55 ±0.05 eV and 1.21 eV, which may act as "dark" centers. Degradation has been interpreted as a result of the instability of dopant in the p-layer. Breaking of Mg-H complexes, with subsequent formation of Mg-H2 and Mg-H-N can in fact explain both the active doping decrease (resulting in increased p-layer resistivity, higher forward voltage and series resistance, and increasing current crowding), and the occurrence of deep levels.