Thermal and efficiency droop in InGaN/GaN light-emitting diodes: decoupling multiphysics effects using temperature-dependent RF measurements

Abstract

Multiphysics processes such as recombination dynamics in the active region, carrier injection and transport, and internal heating may contribute to thermal and efficiency droop in InGaN/GaN light-emitting diodes (LEDs). However, an unambiguous methodology and characterization technique to decouple these processes under electrical injection and determine their individual roles in droop phenomena is lacking. In this work, we investigate thermal and efficiency droop in electrically injected single-quantum-well InGaN/GaN LEDs by decoupling the inherent radiative efficiency, injection efficiency, carrier transport, and thermal effects using a comprehensive rate equation approach and a temperature-dependent pulsed-RF measurement technique. Determination of the inherent recombination rates in the quantum well confirms efficiency droop at high current densities is caused by a combination of strong non-radiative recombination (with temperature dependence consistent with indirect Auger) and saturation of the radiative rate. The overall reduction of efficiency at elevated temperatures (thermal droop) results from carriers shifting from the radiative process to the non-radiative processes. The rate equation approach and temperature-dependent pulsed-RF measurement technique unambiguously gives access to the true recombination dynamics in the QW and is a useful methodology to study efficiency issues in III-nitride LEDs.

Figure 1

(a) Energy band diagram of the LED under flat-band conditions at a current density of 1 kA/cm² simulated using SiLENSe. The original bandgap is reduced to show a more detailed picture of the bandgap and quasi-fermi levels. Dominant carrier processes with their associated rates are shown. (b) Equivalent electrical circuit of the LED.

Figure 2

(a) RF setup used to measure the modulation response and input impedance of the LED. The optical microscope image of the LED under RF probe is included. (b) Concept of the pulsed-RF measurement developed to minimize the effect of self-heating on the measurements. A narrowband filter of the NA is used to pick the main harmonic of the RF frequency among all the generated harmonics.

Figure 3

(a) The DLT extracted from simultaneous fitting of the measured input impedance and modulation response to the input impedance and modulation response of the equivalent circuit of the LED (Fig. 1(b)) and (b) relative EQE of the LED as a function of current density at different stage temperatures.

Figure 4

(a) Injection efficiency, (b) carrier density in the QW, (c) total carrier recombination lifetime in the QW, and (d) radiative efficiency as a function of current density at different stage temperatures. The inset of (b) shows the carrier density vs. current density in linear-linear scale.

Figure 5

(a) Radiative and (b) non-radiative lifetimes, (c) is the non-radiative lifetime at carrier densities of 9 × 10¹⁶, 1.9 × 10¹⁷, and 4.9 × 10¹⁷ cm⁻³ and the fitting of Eq. (5) for various stage temperatures. (d) Radiative and (e) non-radiative recombination rates as a function of carrier density for different stage temperatures. The low slope of the non-radiative rate as a function of carrier density at low carrier densities is due to SRH recombination while the high slope at carrier densities above 10¹⁸ cm⁻³ is attributed to Auger recombination. The inset of (d,e) show the radiative and non-radiative rates vs. carrier density in linear-linear scale. (f) Non-radiative recombination rate as a function of temperature for current densities of 1 to 10 kA/cm².