Study of the strain relaxation in InGaN/GaN multiple quantum well structures

Abstract

The relationship between strain relaxation and quantum well number in InGaN/GaN multiple quantum well (MQW) structures has been investigated by x-ray diffraction (XRD) and low-temperature photoluminescence (PL) measurements. At low temperature, the PL emission energy of a MQW monotonically decreases with increasing quantum well number, and the large pumping induced blueshift of the emission energy decreases rapidly with an increasing quantum well number. Based on an excitation power dependent PL measurement and a previous calculation that was made assuming a fully strained InGaN/GaN MQW, it is found that only a two-period MQW shows no strain relaxation. Furthermore, the residual strain in a MQW with different quantum well numbers is estimated, which is used for an x-ray diffraction kinetic simulation. The simulation agrees well with our measured XRD data, which in turn supports the conclusion based on the PL measurement. Our results indicate that the strain relaxation starts from a three-period InGaN/GaN MQW. Since the strain relaxation generally has a strong influence on the performance of the optical devices, the results presented should definitely be taken into account in designing InGaN/GaN MQW based optical devices.