  We report on the observation of carrier-diffusion-induced defect emission at
high excitation density in a-plane InGaN single quantum wells. When increasing
excitation density in a relatively high regime, we observed the emergence of
defect-related emission together with a significant reduction in bandedge
emission efficiency. The experimental results can be well explained with the
density-activated carrier diffusion from localized states to defect states.
Such a scenario of density-activated defect recombination, as confirmed by the
dependences of photoluminescence on the excitation photon energy and
temperature, is a plausible origin of efficiency droop in a-plane InGaN
quantum-well light-emitting diodes.
