InGaN red LEDs are the key devices to realize AR and VR displays. We have developed InGaN single-QW red LEDs. The InGaN red LEDs show good I-V characteristics. The built-in voltage was only Vb=2.4 V. The peak wavelength at 2.4 V was 641 nm, therefore, the ratio of the photon energy to eVb was as high as 80%. Even at the 20 mA operation, the necessary forward bias was as low as 2.96 V. The light output power density was as high as 0.87 W/cm2 at 20 mA. Therefore, it shows a high WPE of 2.9% at 20 mA.
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.
Fabrication of indium tin oxide (ITO) was optimized for InGaN-based amber/red light-emitting diodes (LEDs). A radiofrequency sputtering reduced the sheet resistivity of ITO at low pressures, and a subsequent two-step annealing resulted in a low sheet resistivity (below 2×10-4 Ωcm) and high transmittance (over 98%) in the amber and red regions between 590 nm to 780 nm. Double ITO layers by sputtering could form an excellent ohmic contact with p-GaN. Application of the double ITO layers on amber and red LEDs enhanced light output power by 15.6% and 13.0%, respectively, compared to those using ITO by e-beam evaporation.
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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