Origin of efficiency droop in GaN-based light-emitting diodes

TL;DR: In this paper, the efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes was investigated and it was shown that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MqW region.

Abstract: The efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.

Summary

Summary

• Where Bn 2 and An are the radiative and nonradiative recombination rates, respectively, and n is the free carrier concentration.
• In order to investigate the physical origin of droop, simulations of the high-power blue LED structure described earlier are performed using APSYS modeling software.
• As a result of the polarization charges, the conduction band slopes upward as it approaches the active region from the n side of the device; this shape is repeated in each of the quantum barriers.
• The conduction band on the n side is higher than the conduction band on the p side, which results in a large electron leakage current ͑as much as 60% of the total current, as will be discussed below͒.
• Figure 3 plots the internal quantum efficiency ͑IQE͒ and the electron leakage current across the EBL as a function of the forward current for the reference LED and LEDs in which the AlGaInN MQW quantum barriers and EBL are polarization matched to the QWs and to GaN, respectively.
• When an AlGaInN EBL is polarization matched to GaN, the light output increases by 49.5% at 350 mA and the droop decreases to 22%.
• Electron leakage still constitutes more than 40% of the total injected current, due to the remaining polarization charges in the MQW.
• Thus, polarization fields in the MQW active region and the EBL are the physical origin of the efficiency droop occurring in GaInN LEDs.
• To determine the bandgap of a quaternary alloy, three bandgap terms-based on AlN, GaN, and InN-are calculated.
• EBL2 maintains the bandgap of the reference EBL but reduces the polarization charge to 50% rather than eliminating it.
• Therefore, even aluminum-free barrier compositions, such as QB5, could provide an enhancement of efficiency and reduction of droop.

Figures

FIG. 4. Color online Bandgap dashed-line contours and total polarization charge solid-line contours of AlGaInN as a function of Al and In compositions.

FIG. 3. Color online IQE and leakage current ratio of GaInN/GaN and GaInN/AlGaInN LEDs with and without polarization effect in the MQW and/or the EBL.

FIG. 2. Color online Calculated band diagram of reference GaInN/GaN LED as well as GaInN/AlGaInN LED structure with polarization-matched MQW under a forward bias condition.

FIG. 1. Color online a EQE of GaInN MQW LED at heat-sink temperatures of 25–150 °C vs current. b EQE measured by integrating sphere and MQW radiative efficiency of GaInN wafer vs optical carrier generation rate.

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In this paper, the efficiency droop in GaInN/GaN multiple-quantum well MQW light-emitting diodes is investigated.