For the development of efficient red LEDs with high-In-content InGaN quantum wells (QWs), we have developed the micro-flow-channel MOVPE method. This MOVPE can grow high-In-content InGaN at higher growth temperatures, resulting in higher quality. Also, we have introduced the strain compensation method at the QW region. Barrier layers consisting of Al(Ga)N could compensate for a compressive strain induced by InGaN. The strain compensation method has improved LED efficiency and elongated peak EL electroluminescence.
We have developed highly efficient InGaN red LEDs via the strain-compensated InGaN SQW structure. The LED structure is an InGaN single-quantum-well with AlGaN barriers to compensate for the compressive strain in the InGaN well layer. The red LEDs exhibited an EQE of 4.3%, a light output of 1.7 mW, and a wavelength of 621 nm at 20 mA (10 A/cm2) under 2.96 V. We applied the hydrogen passivation method to pixelize the planar-type micro-LEDs and to fabricate the efficient mesa-type micro-LEDs by suppressing the carrier surface recombination.
The LEDs were obtained that the peak emission wavelength and FWHM were 665 nm and 67 nm at 20 mA, respectively. It exhibited a large blueshift of the EL peak wavelength from 691 nm at 5 mA to 631 nm at 100 mA. In this range, the blue-shifted value was 60 nm. Besides, we realized the single peak emission LEDs without an additional emission. We obtained a light output, forward voltage, and EQE of 0.07 mW, 2.45 V, and 0.19% at 20 mA, respectively. The LEDs exhibited the temperature stability of EL intensity and peak wavelength.
We obtained the EL intensity enhancement by a factor of 1.3 with increasing of n-GaN thickness from 2 to 8 µm. We achieved a light output, forward voltage, FWHM and external quantum efficiency of 0.64 mW, 3.3 V, 59 nm, and 1.6% at 20 mA, respectively. Particularly, the wall plug efficiency was 1.0%, which is comparable with the state-of-the-art InGaN-based red LEDs. The reduction of the in-plane compressive stress by the GaN underlying layers appears to be crucial for enhancing the light output of InGaN-based red LEDs on conventional sapphire substrates.
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