Degradation-induced low frequency noise and deep traps in GaN/InGaN near-UV LEDs

Electrical stressing of near-UV (peak wavelength 390-395 nm) multi-quantum-well GaN/InGaN light emitting diodes at a high drive current of 650 mA and elevated temperature of 110 °C causes a significant degradation in external quantum efficiency (EQE), correlated with the formation of nitrogen interstitial-related electron traps at E_c - 0.8 eV. The dependence of the spectral density of current noise S_I on forward current I_f showed two regions prior to accelerated aging, with S_I ∼ I_f due to the current flow via localized leakage channels (presumably dislocations) and S_I ∼ I_f ² related to the generation-recombination noise caused by the E_c - 0.8 eV states and E_v + 0.75 eV hole traps in the space charge region. Electrical stress for <922 h did not change the EQE but gradually increased both reverse and forward leakage current. This was accompanied by a gradual increase in the density of the hole traps, but not the electron traps. The mechanism appears to be the displacement of Ga and In atoms, with the interstitials decorating dislocations and forming local leakage channels. For stress times >922 h, the peak EQE decreased from 26% to 15% and was accompanied by a further increase in the leakage current and density of both types of traps. One of the 20 studied diodes showed an anomalously high forward leakage current, and the noise spectrum in it was dominated by the S_I ∼ I_f ⁴ region typical for the presence of local overheated areas (presumably local In composition fluctuations). The EQE of this sample began to degrade after a much shorter stress time of 258 h.