Improved Emission Efficiency in InGaN / GaN Quantum Wells with Compositionally-Graded Barriers Studied by Time-Resolved Photoluminescence Spectroscopy

Recombination dynamics in InGaN/GaN multiple quantum wells (MQWs) having different well/barrier potential profiles were studied. Time-resolved photoluminescence (TRPL) signals of the MQWs having regular potential profiles (rectangular MQW) and those having the compositionally-graded barriers (trapezoidal MQW) exhibited similar stretched exponential decay, which is a fingerprint for localized exciton emissions. The luminescence lifetimes (τ_PL) of them were as long as 4-10ns between 8 and 300 K, indicating the reduced wavefunction overlap due to the internal polarization fields. According to the analysis based on the model three-level scheme, little difference was found in the nonradiative lifetimes in the free / extended states between the two MQWs. However, the increase of the combined transfer and radiative lifetime with the increase in temperature from 8 to 300 K in the trapezoidal MQW was suppressed by a factor of 1.5 compared to that in the rectangular one, reflecting the recovery of wavefunction overlap and effectively larger localization depth due to the reduced effective field. As a result, the quantum efficiency of the trapezoidal MQW was improved by 40% compared to that of the rectangular one at 300 K.