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.
We have proposed an efficiency droop model which can comprehensively explain experimental IQE droop phenomena
occurring at different temperatures, materials, and active structures. In our model, carriers are located and recombined
both radiatively and nonradiatively inside randomly distributed In-rich areas of InGaN-based QWs and the IQE droop
originates from the saturated radiative recombination rate and the monotonically increasing nonradiative recombination
rate there. Due to small effective active volume and small density of states of In-rich areas, carrier density is rapidly
increased even at low current density and the radiative recombination rate is easily saturated by different distributions of
electrons and holes in the momentum k-space. A measurement method that can separately estimate the radiative and
nonradiative carrier lifetimes just at room temperature is theoretically developed by analyzing the time-resolved
photoluminescence (TRPL) response. The method is applied to a blue InGaN/GaN QW LED. The experimental results
show that the radiative carrier lifetime increases and the nonradiative carrier lifetime saturates with increasing TRPL
laser power, which is one of direct evidences validating our IQE droop model.
High quality epitaxial growth and uniform current spreading are essential to III-nitride light emitting diodes (LEDs) for
superior wall-plug efficiency and reliability. An analysis method of current spreading based on 3-dimensional circuit
modeling is introduced. We have investigated influences of the current spreading in the lateral-electrode type blue LEDs
of 320 × 320 μm2 size theoretically and experimentally. It is known that the current spreading can be greatly reduced by careful design of electrode pattern. Uniform current spreading is very important to improve electrical and optical
characteristics such as series resistance, efficiency droop, leakage current with operational time leakage current, and
electrostatic discharge (ESD) voltage. A method improving ESD voltage is presented by inserting floating metal near the n-electrode. About 4 times larger ESD voltages are experimentally measured at LEDs with floating metal compared to conventional LEDs without one. The internal quantum efficiency (IQE) is the most important factor affecting overall LED performances. A measurement method of the IQE measurable just at room temperature is proposed and demonstrated. The method utilizes both the time responses of the time-resolved photoluminescence (TRPL) as a function of the excitation femtosecond laser pumping power and their theoretical analysis based on the carrier rate equation.
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