On the efficiency droop in InGaN multiple quantum well blue light emitting diodes and its reduction with p-doped quantum well barriers

Multiple quantum well (MQW) InGaN light emitting diodes with and without electron blocking layers, with relatively small and large barriers, with and without p-type doping in the MQW region emitting at similar to 420 nm were used to determine the genesis of efficiency droop observed at injection levels of approximately >= 50 A/cm(2). Pulsed electroluminescence measurements, to avoid heating effects, revealed that the efficiency peak occurs at similar to 900 A/cm(2) current density for the Mg-doped barrier, near 550 A/cm(2) for the lightly doped n-GaN injection layer, meant to bring the electron injection level closer to that of holes, and below 220 A/cm(2) for the undoped InGaN barrier cases. For samples with GaN barriers (larger band discontinuity) or without p-AlGaN electron blocking layers the droop occurred at much lower current densities (<= 110 A/cm(2)). In contrast, photoluminescence measurements revealed no efficiency droop for optical carrier generation rates corresponding to the maximum current density employed in pulsed injection measurements. All the data are consistent with heavy effective mass of holes, low hole injection efficiency (due to relatively lower p-doping) leading to severe electron leakage being responsible for efficiency droop. (C) 2008 American Institute of Physics.