Abstract
Light-emitting diode (LED) efficiency has attracted considerable interest because of the extended use of solid-state lighting. Owing to lack of direct measurement, identification of the reasons for efficiency droop has been restricted. A direct measurement technique is developed in this work for characterization of biaxial stress in GaN-based blue LEDs under electrical injection. The Raman shift of the GaN E2 mode evidently decreases by 4.4 cm−1 as the driving current on GaN-based LEDs increases to 700 mA. Biaxial compressive stress is released initially and biaxial tensile stress builds up as the current increases with respect to the value of stress-free GaN. First-principles calculations reveal that electron accumulation is responsible for the stress variation in InxGa1−xN/GaN quantum wells, and then reduces the transition probability among quantum levels. This behavior is consistent with the measured current-dependent external quantum efficiency. The rule of biaxial stress-dependent efficiency is further validated by controlling the biaxial stress of GaN-based LEDs with different sapphire substrate thicknesses. This work provides a method for direct observation of the biaxial stress effect on efficiency droop in LEDs under electrical injection.
Highlights
GaN-based light-emitting diodes (LEDs) are revolutionizing lighting applications toward realizing high efficiency
Raman scattering produces two possible outcomes: one appears in the lower energy side of the excitation laser line, which is called Stokes Raman scattering; and another is exhibited in the higher energy side, which is called anti-Stokes Raman scattering
Extensive studies have focused on the quantum efficiency of LEDs
Summary
GaN-based light-emitting diodes (LEDs) are revolutionizing lighting applications toward realizing high efficiency. Direct measurement of Auger electrons emitted from a semiconductor LED under electrical injection has been proposed recently to observe the signature of Auger electrons at high injected current densities higher than 50 A/cm[2,7] which identifies the dominant efficiency droop mechanism[17]. The effect means that lattice stress is changed because of electron injection Both temperature- and current-dependent lattice constants would affect electronic structures, transition probability, and even quantum efficiency. The stress variations are found to be attributed to the current injection based on the comparison of the simulated results with the stress variations and efficiency droop under different currents, and reduction of the hole quantum states.
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