Rapid efficiency roll-off in high-quality green light-emitting diodes on freestanding GaN substrates

InGaN/GaN multiple-quantum-well green light-emitting diodes (LEDs) were grown on freestanding GaN and sapphire substrates. The density of microstructural defects in the LED on GaN was substantially reduced, leading to a significant reduction in defect-assisted tunneling currents and an improved injection efficiency under low bias. The LED on GaN outperformed the LED on sapphire at low injection currents and exhibited a similar to 65% peak internal quantum efficiency. However, it suffered from even more dramatic efficiency roll-off, which occurs at a current density as low as 0.3 A/cm(2). This behavior is explained as the combined result of efficient current injection and significant carrier overflow in a high-quality LED.