Efficiency Droop Improvement in InGaN/GaN Light-Emitting Diodes by Graded-Composition Multiple Quantum Wells

InGaN/GaN light-emitting diodes (LEDs) with a graded-composition multiple quantum well (GQW) were designed not only to investigate the effect of polarization field on the efficiency droop in InGaN/GaN multiple quantum wells (MQWs) but to find out possible solutions to prevent or reduce the efficiency droop in GaN-based LEDs as well. Pulsed electroluminescence measurement, to avoid heating effects partly, revealed that the output intensity peak occurs at
$\sim\! 280\ \hbox{A/cm}^{2}$
current density for the GQW structure, whereas for a conventional structure, it occurs at a much lower current density of
$\sim\!\! 150\ \hbox{A/cm}^{2}$
. The relative quantum efficiency indicates that the efficiency droop in GQW LEDs was reduced effectively due to the reduction in polarization field in the active layer. In the test GQW structure with inverse indium composition variation direction, electrons escape from the active region, which is in company with rapid saturation of output power and decrease in efficiency observed at high current density. It indicates that at high current density, the electron leakage is obvious. These results suggest that the polarization field in the active layer and the consequent electron leakage are probably the main mechanism responsible for the efficiency droop at high current injection levels.