InGaN Single-Quantum-Well LEDs

Abstract

As discussed in previous sections, recent research on III-V nitrides has paved the way for the realization of high-quality crystals of A1GaN and InGaN, and p-type conduction in AlGaN. The hole-compensation mechanism of p-type A1GaN has also been elucidated. High-brightness blue and blue-green light emitting diodes (LEDs) with a luminous intensity of 2 cd have been fabricated using these techniques and are now commercially available. In order to obtain blue and blue-green emission centers in these InGaN/A1GaN double-heterostructure (DH) LEDs, Zn doping of the InGaN active layer was performed. Although these InGaN/A1GaN DH LEDs produce a high-power light output in the blue and blue-green region with a broad emission spectrum (full width at half-maximum (FWHM = 70 nm), green or yellow LEDs with a peak wavelength longer than 500 nm have not been fabricated. The longest peak wavelength of the electroluminescence (EL) of InGaN/A1GaN DH LEDs achieved thus far is 500 nm (blue-green) because the crystal quality of the InGaN active layer of DH LEDs becomes poor when the indium mole fraction is increased to obtain a green band-edge emission. On the other hand, in conventional green GaP LEDs the external quantum efficiency is only 0.1% due to the indirect band-gap and the peak wavelength is 555 nm (yellowish green) [304]. As another material for green emission devices, AllnGaP has been used. The present performance of green AlInGaP LEDs is an emission wavelength of 570 nm (yellowish green) and maximum external quantum efficiency of 1% [304]. When the emission wavelength is reduced to the green region, the external quantum efficiency drops sharply because the band structure of AlInGaP approaches an indirect transition. Therefore, high-brightness pure green LEDs, which have a high efficiency of above 1% at the peak wavelength of 510–530 nm with a narrow FWHM, are not yet commercially available.