This work investigates the influence of residual stress on the performance of InGaN-based red light-emitting diodes (LEDs) by changing the thickness of the underlying n-GaN layers. The residual in-plane stress in the LED structure depends on the thickness of the underlying layer. Decreased residual in-plane stress resulting from the increased thickness of the underlying n-GaN layers improves the crystalline quality of the InGaN active region by allowing for a higher growth temperature. The electroluminescence intensity of the InGaN-based red LEDs is increased by a factor of 1.3 when the thickness of the underlying n-GaN layer is increased from 2 to 8 μm. Using 8-μm-thick underlying n-GaN layers, 633-nm-wavelength red LEDs are realized with a light-output power of 0.64 mW and an external quantum efficiency of 1.6% at 20 mA. The improved external quantum efficiency of the LEDs can be attributed to the lower residual in-plane stress in the underlying GaN layers.

InGaN-based LEDs are efficient light sources in the blue–green light range and have been successfully commercialized in the last decades. Extending their spectral range to the red region causes a significant reduction in LED efficiency. This challenge hinders the integration of red, green, and blue LEDs based on III-nitride materials, especially for full-color micro-LED displays. We review our recent progress on InGaN-based red LEDs with different chip sizes from hundreds to tens of micrometers, including the epitaxial structures, device fabrication, and optical performance (peak wavelength, full-width at half-maximum, light output power, efficiency, temperature stability, and color coordinates).

Here we report InGaN-based red light-emitting diodes (LEDs) grown on ( 2 ¯ 01 ) β-Ga₂O₃ substrates. AlN/AlGaN strain-compensating layers and hybrid multiple-quantum-well structures were employed to improve the crystalline-quality of the InGaN active region. A bare LED showed that peak wavelength, light output power, and external quantum efficiency were 665 nm, 0.07 mW, and 0.19% at 20 mA, respectively. As its forward voltage was 2.45 V at 20 mA, the wall-plug efficiency was 0.14%. The characteristic temperature of the LEDs was 222 K at 100 mA evaluated from the temperature dependence of electroluminescence.

We investigated the performance of InGaN-based red/green micro-light-emitting diodes (µLEDs) ranging from 98 × 98 µ m 2 to 17 × 17 µ m 2 . The average forward voltage at 10 A / c m 2 was independent of the dimension of µLEDs. Red µLEDs exhibited a larger blueshift of the peak wavelength ( ∼ 35 n m ) and broader full-width at half maximum ( ≥ 50 n m ) at 2 − 50 A / c m 2 compared to green µLEDs. We demonstrated that 47 × 47 µ m 2 red µLEDs had an on-wafer external quantum efficiency of 0.36% at the peak wavelength of 626 nm, close to the red primary color defined in the recommendation 2020 standard.

We demonstrated 10 × 10 arrays of InGaN 17 μm × 17 μm micro-light-emitting diodes (μLEDs) with a peak wavelength from 662 to 630 nm at 10 – 50 A / cm 2 . The on-wafer external quantum efficiency reached 0.18% at 50 A / cm 2 . The output power density of the red μLEDs was obtained as 1.76 mW / mm 2 , which was estimated to be higher than that of 20 μm × 20 μm AlInGaP red μLEDs ( ∼ 630 nm ). Finally, we demonstrate that InGaN red/green/blue μLEDs could exhibit a wide color gamut covering 81.3% and 79.1% of the Rec. 2020 color space in CIE 1931 and 1976 diagrams, respectively.