High crystal quality in InGaN-based green light-emitting diodes with superlattice strain-relaxation layer: Characterization and efficiency improvement strategy

High indium composition in green-light InGaN/GaN light-emitting diodes (LEDs) can lead to significant efficiency degradation, which has not been comprehensively resolved. We made a systematic investigation of high efficiency green InGaN LEDs with the insertion of superlattices (SLs) and their epitaxial wafers. We elucidated the mechanistic role of SLs in suppressing the localization effect mainly by temperature-dependent photoluminescence (PL). Further corroboration of the good crystalline quality is provided via power-dependent PL and thermal admittance spectroscopy (TAS). The homogeneity of indium composition could suppress the Shockley-Read-Hall recombination significantly: a radiative efficiency of more than 44.7% at 300 K was achieved, and no identifiable defect levels were found by TAS. However, the overall radiative efficiency was still limited due to electron crowding in the quantum well closest to the p-type region and inadequate hole injection, illustrated by the capacitance-voltage measurements and the simulation results with the finite element method. Our analysis along with the introduced improvement strategy could provide relatively comprehensive guidance for subsequent designs.