Abstract
The development of efficient green light-emitting diodes (LEDs) is of paramount importance for the realization of colour-mixing white LEDs with a high luminous efficiency. While the insertion of an InGaN/GaN superlattice (SL) with a lower In content before the growth of InGaN/GaN multiple quantum wells (MQWs) is known to increase the efficiency of LEDs, the actual mechanism is still debated. We therefore conduct a systematic study and investigate the different mechanisms for this system. Through cathodoluminescence and Raman measurements, we clearly demonstrate that the potential barrier formed by the V-pit during the low-temperature growth of an InGaN/GaN SL dramatically increases the internal quantum efficiency (IQE) of InGaN quantum wells (QWs) by suppressing non-radiative recombination at threading dislocations (TDs). We find that the V-pit potential barrier height depends on the V-pit diameter, which plays an important role in determining the quantum efficiency, forward voltage and efficiency droop of green LEDs. Furthermore, our study reveals that the low-temperature GaN can act as an alternative to an InGaN/GaN SL structure for promoting the formation of V-pits. Our findings suggest the potential of implementing optimized V-pits embedded in an InGaN/GaN SL or low-temperature GaN structure as a beneficial underlying layer for the realization of highly efficient green LEDs.
Highlights
Due to the large lattice constant and thermal expansion coefficient mismatches between GaN and the sapphire substrate, the threading dislocation (TD) density in GaN-based light-emitting diodes (LEDs) is on the order of 108 cm−215–20
We found that the V-pit potential barrier height around the TD is determined by the size of the V-pit that plays an important role in determining the quantum efficiency, forward voltage and efficiency droop of green LEDs
We have demonstrated the effects of V-pits on electronic and optical properties and efficiency droop of green LEDs
Summary
Due to the large lattice constant and thermal expansion coefficient mismatches between GaN and the sapphire substrate, the threading dislocation (TD) density in GaN-based LEDs is on the order of 108 cm−215–20. Practical questions remain unanswered that are critical to device design and optimization, such as how V-pits affect the electronic and optical properties and efficiency droop of green LEDs. In this study, we unambiguously demonstrate that the potential barrier formed by the V-pit dramatically increases the internal quantum efficiency (IQE) of the InGaN QWs by suppressing non-radiative recombination of the carriers at the TDs. We have analysed the effects of nanometre-scale V-pits on the electronic and optical properties and efficiency droop of green LEDs using various periods of InGaN/GaN SLs and low-temperature GaN. These findings, obtained from green LEDs with various underlying layer configurations, including InGaN/GaN SL and low-temperature GaN, reveal several unexpected effects and are important for future device optimization
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