Thermal droop investigations on ultraviolet C-band light-emitting diodes

Abstract

We report on an exceptional thermal droop of 3.3% for a 272 nm ultraviolet C-band (UVC) light-emitting diode (LED) at 300 mA (39.4 A/cm2) and 102 °C junction temperature. To understand the superior thermal droop, we developed a method to determine the contributions of Shockley–Read–Hall (SRH) non-radiative, bimolecular radiative, and Auger non-radiative recombinations. Our analysis shows that the small thermal droop of the LED is due to a low SRH non-radiative recombination efficiency and a high defect thermal activation energy of 456.3 meV. At room temperature, the deciphered recombination efficiencies for the 272 nm LED, across a driving current range of 50–300 mA, are 1.0%–0.3% for SRH non-radiative recombination, 74.5%–60.2% for bimolecular radiative recombination, and 24.4%–39.5% Auger non-radiative recombination. We also compared a 268 nm LED that showed a significant thermal droop of 33% (at 350 mA and 105 °C) but a minimal injection-dependent efficiency droop of 2.2% (at 350 mA). Our method produced a defect thermal activation energy of 308.4 meV for this LED. The room-temperature recombination efficiencies for the 268 nm LED (50–350 mA) are 11.4%–14.3% for SRH non-radiative recombination, 80.0%–81.8% for bimolecular radiative recombination, and 5.7%–6.9% for Auger non-radiative recombination. This study suggests that thermal droop is primarily caused by SRH non-radiative recombination, while injection-dependent efficiency droop is mainly due to Auger recombination. These factors are linked to the material quality and the polarization electric field within the light-emitting quantum wells. Optimal UVC LED performance, with minimal thermal and injection-dependent droop, requires both excellent material quality and a low polarization field.