Jongil Hwang

Development of InGaN-based red LED grown on (0001) polar surface

We report on the optical properties of the InGaN-based red LED grown on a c-plane sapphire substrate. Blue emission due to phase separation was successfully reduced in the red LED with an active layer consisting of 4-period InGaN multiple quantum wells embedding an AlGaN interlayer with the Al content of 90% on each quantum well. The light output power and external quantum efficiency at a dc current of 20 mA were 1.1 mW and 2.9% with the wavelength of 629 nm, respectively. This is the first demonstration of a nitride-based red LED with the light output power exceeding 1 mW at 20 mA.

Effects of local structure on optical properties in green-yellow InGaN/GaN quantum wells

We have performed photoluminescence measurements (PL) for green-yellow InGaN/GaN multiple quantum wells (MQWs) in which InGaN layer was embedded in the barrier as a strain-controlling interlayer to verify the effect on the crystal quality and the internal electric field (F0). From the analysis of both the time-resolved PL and the excitation-power dependence of PL, it has been revealed that the PL intensities and the decay time were enhanced for the MQWs with the InGaN interlayer although the wavelength dependence of the F0 scarcely changes. This indicates that the InGaN interlayer suppresses the degradation of InGaN quantum well rather than the quantum- confinement Stark effect, suggesting that optical properties can be improved by improving the crystal quality through optimizing the local structure as well as the growth conditions. From the light-emitting diodes using the interlayer, we obtained an output power and external quantum efficiency of 10.2 mW and 23.5% at 20 mA with the wavelength of 552 nm.

Green, Yellow, and Red LEDs

A study to overcome the “green gap” problem is introduced. We show a breakthrough achieved by two-step growth with adopting AlGaN interlayer in this chapter. Bright yellow, amber with red LEDs and an application of phosphor-free white LED are demonstrated.