In InGaN/GaN blue light-emitting diodes (LEDs) widely utilized for general lighting, there exist various material issues that lead to the unwanted nonradiative recombination. In this paper, we utilize various characterization techniques to investigate the nonradiative recombination mechanisms in LED devices. With the characterization techniques such as temperature-dependent external quantum efficiency, current-voltage, and electroluminescence spectra, we show that different nonradiative recombination processes such as the Shockley-Read-Hall recombination and the defect-assisted tunneling can play roles in the LED devices. Information on the dominant nonradiative recombination obtained by these analyses can be used for further improving the quantum efficiency of the device.
The efficiency droop in light-emitting diodes (LEDs) represents a gradual decrease of the internal quantum efficiency
(IQE) with increasing current. Experimentally, the IQE droops are strong functions of material, epitaxial and chip
structures, and operating temperature. Recently, we have proposed an IQE droop model as the saturation of the radiative
recombination rate at low current and subsequent increase in the nonradiative recombination rates at high current. Once
the radiative recombination rate begins to saturate at an active region, the carrier density as well as the nonradiative
recombination rate rapidly increase there. Eventually, the IQE droop appears from the increase in the nonradiative
recombination rate being much larger than that in the radiative one. A dominant nonradiative recombination process is
not solely determined for each LED chip, but it could vary with current level and operating temperature. As temperature
decreases, in general, the IQE droop becomes larger with the peak IQE occurring at an extremely small current level. We
test the droop model by investigating the radiative and nonradiative recombination processes separately from the
cryogenic to room temperature. The characterization methods include comparative efficiency study between
photoluminescence (PL) and electroluminescence (EL), open-circuit voltage under resonant PL excitation, interrelations
of current-voltage-light characteristics, and EL spectra of color-coded quantum wells (QWs). Although a sudden increase
of the nonradiative recombination rate is an apparent cause of the IQE droop, the saturation of the radiative
recombination rate is the common trigger behind the IQE droop issue.