Electrical and optical characteristics of green light-emitting diodes grown on bulk GaN substrates

InGaN-based LEDs suffer from a significant drop in quantum efficiency (QE) under high-current operation. We studied the Electroluminescence (EL) of InGaN-based multiple-quantum-well green LEDs on both Sapphire and free standing Bulk GaN, in an attempt to shed light on the underlying mechanism for the efficiency droop problem. The density of microstructural defects in the LED on GaN was substantially reduced, leading to a significant reduction in defectassisted 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 ~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/cm2. The EQE roll-off is mitigated when the LEDs were tested at elevated temperatures. These results are explained as the combined result of efficient current injection and significant carrier overflow in a high-quality LED.