Efficiency of GaN light-emitting diodes and carrier energetic relaxation in InGaN/GaN quantum wells

A direct correlation is found between efficiency droop and changes in the high energy side of emission spectrum of InGaN/GaN quantum wells (QWs) with increasing current density. A dynamic model of energy relaxation of injected carriers in exponential band tails of InGaN QWs is proposed. At low injection level the carriers trapped by shallow tail states quickly hop directly to lower-energy tail states. This results in the strong carrier localization and high-energy cutoff of emission spectrum. At higher injection level hopping directly to deeper states is suppressed due to the partial filling of tail states. As a result the ratio of mobile to localized carriers increases with current. The emission efficiency decreases because of the enhancement of lateral diffusion length, carrier capture by defects and tunnel-recombination leakage via defects. Simultaneously the high-energy cutoff of emission spectrum shifts to higher energy. Thus, the magnitude of efficiency droop is directly related to the relative broadening of emission spectrum.