Impact of carrier localization and diffusion on photoluminescence in highly excited cyan and green InGaN LED structures

Localization of charge carriers is of crucial importance in InGaN light emitting devices since it governs carrier transport and ensures high radiative efficiency. In this work, we observe the dynamics of carrier localization from an untypical redshift of photoluminescence spectra. We investigate two light emitting diode structures grown by MOCVD on c-plane sapphire and emitting at 500nm (cyan) or 530nm (green). For the study, we employ photoluminescence, differential transmission, and light induced transient gratings techniques. We observe non-monotonous dynamics of photoluminescence peak position: a blueshift at short delay times (<10ns) is later replaced by a redshift, which increases both with time and excitation. The redshift values as large as 30meV in the cyan and 20meV in the green structure were observed at 40ns delay. We attribute this redshift to density-dependent and diffusion-driven carrier redistribution between the shallower and deeper localized states. The carrier delocalization with increasing density is confirmed by growing diffusivity of carriers with excitation. We also demonstrate a correlation between the growth of diffusion coefficient and the onset of efficiency droop.