What is red? On the chromaticity of orange-red InGaN/GaN based LEDs

Abstract

The full width at half maximum (FWHM) of the luminescence of visible InGaN quantum well (QW) based emitters increases with wavelength. This broadening of the luminescence decreases the color saturation from 100% to about 70% up a wavelength shorter than 515 nm. For emission wavelengths longer than 515 nm, the saturation surprisingly increases again and reaches 95% beyond 560 nm. More important, in the yellow-red range, the perceived hue is strongly blue-shifted by a broad emission. This phenomenon is known as Abney effect, and it originates from the spectral sensitivities of the cone cells of the human eyes. As a consequence, for red InGaN QW based LEDs, the peak wavelength must be even further in the red. Based on a large set of QWs grown on different crystal orientations, we correlated the wavelength and the luminescence FWHM of InGaN/GaN QWs to build a model which predicts the perceived chromaticity, i.e., the apparent hue and saturation of yellow-red nitride LEDs. We also applied this model to (11-22), a-plane, and m-plane QWs and compared our data to the state-of-the-art of the literature. We concluded that the FWHM of the luminescence is a critical parameter to design and a further challenge for red InGaN-based light-emitting diodes.