A well protection layer as a novel pathway to increase indium composition : a route towards green emission from a blue InGaN/GaN multiple quantum well

We have investigated the effects of a well protection layer (WPL) on the optical and crystal properties of an InGaN/GaN multiple quantum well (MQW). The five-pair MQW, consisting of an InGaN well grown at 750 °C and a GaN barrier grown at 850 °C, was simply embedded between GaN cladding layers on a sapphire (0001) substrate. While this dual-temperature MQW growth scheme seemed better suited to the GaN barrier quality, it exposed the volatile InGaN well to a higher temperature ambient during the ramping-up process to grow the barrier. In order to prevent damage to the fragile well, a thin GaN WPL was subsequently coated on each well layer at the same temperature before ramping-up the temperature. Consequently, it was found that the WPL directly influenced the indium composition and optical properties of the MQW. The indium composition was in fact increased, as was evident from x-ray diffraction experiments. In addition, photoluminescence measurements showed that the emission peak wavelength was increased from 464 to 520 nm. These results provide evidence that the WPL effectively suppresses indium re-evaporation during the ramping-up time. The present study proposes that the WPL leads to a new way to increase the wavelength of InGaN/GaN MQWs.